Fix SEGV by stack extension in mrb_get_args()

mrb_get_args() keeps pointer of the current stack. But address of the
current stack maybe changed by method call.

'i' format character calls #to_i when the argument isn't integer but
has #to_i.

Here is a code that may call #to_i in mrb_get_args():

    case 'i':
      // ...
            default:
              *p = mrb_fixnum(mrb_Integer(mrb, ARGV[arg_i]));
              break;
     // ...

Here is a code #to_i is called:

    class X
      def initialize(i)
        @i = i
      end

      def to_i
        @i
      end
    end

    [][X.new(0), 0] # X#to_i is called

So, mrb_get_args() shouldn't keep pointer and use it. mrb_get_args()
should always refer mrb->ci->stack to use valid address of the current
stack.

src/class.c

@@ -466,9 +466,10 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
 {
   char c;
   int i = 0;
-  mrb_value *sp = mrb->c->stack + 1;
   va_list ap;
   int argc = mrb->c->ci->argc;
+  int arg_i = 0;
+  mrb_bool array_argv;
   mrb_bool opt = FALSE;
   mrb_bool given = TRUE;
 
@@ -477,8 +478,14 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
     struct RArray *a = mrb_ary_ptr(mrb->c->stack[1]);
 
     argc = a->len;
-    sp = a->ptr;
+    array_argv = TRUE;
+  } else {
+    array_argv = FALSE;
   }
+
+#define ARGV \
+  (array_argv ? mrb_ary_ptr(mrb->c->stack[1])->ptr : (mrb->c->stack + 1))
+
   while ((c = *format++)) {
     switch (c) {
     case '|': case '*': case '&': case '?':
@@ -502,7 +509,7 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
 
         p = va_arg(ap, mrb_value*);
         if (i < argc) {
-          *p = *sp++;
+          *p = ARGV[arg_i++];
           i++;
         }
       }
@@ -515,7 +522,7 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         if (i < argc) {
           mrb_value ss;
 
-          ss = *sp++;
+          ss = ARGV[arg_i++];
           switch (mrb_type(ss)) {
           case MRB_TT_CLASS:
           case MRB_TT_MODULE:
@@ -537,14 +544,14 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         p = va_arg(ap, mrb_value*);
         if (*format == '!') {
           format++;
-          if (i < argc && mrb_nil_p(*sp)) {
-            *p = *sp++;
+          if (i < argc && mrb_nil_p(ARGV[arg_i])) {
+            *p = ARGV[arg_i++];
             i++;
             break;
           }
         }
         if (i < argc) {
-          *p = to_str(mrb, *sp++);
+          *p = to_str(mrb, ARGV[arg_i++]);
           i++;
         }
       }
@@ -556,14 +563,14 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         p = va_arg(ap, mrb_value*);
         if (*format == '!') {
           format++;
-          if (i < argc && mrb_nil_p(*sp)) {
-            *p = *sp++;
+          if (i < argc && mrb_nil_p(ARGV[arg_i])) {
+            *p = ARGV[arg_i++];
             i++;
             break;
           }
         }
         if (i < argc) {
-          *p = to_ary(mrb, *sp++);
+          *p = to_ary(mrb, ARGV[arg_i++]);
           i++;
         }
       }
@@ -575,14 +582,14 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         p = va_arg(ap, mrb_value*);
         if (*format == '!') {
           format++;
-          if (i < argc && mrb_nil_p(*sp)) {
-            *p = *sp++;
+          if (i < argc && mrb_nil_p(ARGV[arg_i])) {
+            *p = ARGV[arg_i++];
             i++;
             break;
           }
         }
         if (i < argc) {
-          *p = to_hash(mrb, *sp++);
+          *p = to_hash(mrb, ARGV[arg_i++]);
           i++;
         }
       }
@@ -597,7 +604,7 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         pl = va_arg(ap, mrb_int*);
         if (*format == '!') {
           format++;
-          if (i < argc && mrb_nil_p(*sp)) {
+          if (i < argc && mrb_nil_p(ARGV[arg_i])) {
             *ps = NULL;
             *pl = 0;
             i++;
@@ -605,7 +612,7 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
           }
         }
         if (i < argc) {
-          ss = to_str(mrb, *sp++);
+          ss = to_str(mrb, ARGV[arg_i++]);
           *ps = RSTRING_PTR(ss);
           *pl = RSTRING_LEN(ss);
           i++;
@@ -620,14 +627,14 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         ps = va_arg(ap, const char**);
         if (*format == '!') {
           format++;
-          if (i < argc && mrb_nil_p(*sp)) {
+          if (i < argc && mrb_nil_p(ARGV[arg_i])) {
             *ps = NULL;
-            i++; sp++;
+            i++; arg_i++;
             break;
           }
         }
         if (i < argc) {
-          ss = to_str(mrb, *sp++);
+          ss = to_str(mrb, ARGV[arg_i++]);
           *ps = mrb_string_value_cstr(mrb, &ss);
           i++;
         }
@@ -644,15 +651,15 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         pl = va_arg(ap, mrb_int*);
         if (*format == '!') {
           format++;
-          if (i < argc && mrb_nil_p(*sp)) {
+          if (i < argc && mrb_nil_p(ARGV[arg_i])) {
             *pb = 0;
             *pl = 0;
-            i++; sp++;
+            i++; arg_i++;
             break;
           }
         }
         if (i < argc) {
-          aa = to_ary(mrb, *sp++);
+          aa = to_ary(mrb, ARGV[arg_i++]);
           a = mrb_ary_ptr(aa);
           *pb = a->ptr;
           *pl = a->len;
@@ -666,8 +673,8 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
 
         p = va_arg(ap, mrb_float*);
         if (i < argc) {
-          *p = mrb_to_flo(mrb, *sp);
-          sp++;
+          *p = mrb_to_flo(mrb, ARGV[arg_i]);
+          arg_i++;
           i++;
         }
       }
@@ -678,13 +685,13 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
 
         p = va_arg(ap, mrb_int*);
         if (i < argc) {
-          switch (mrb_type(*sp)) {
+          switch (mrb_type(ARGV[arg_i])) {
             case MRB_TT_FIXNUM:
-              *p = mrb_fixnum(*sp);
+              *p = mrb_fixnum(ARGV[arg_i]);
               break;
             case MRB_TT_FLOAT:
               {
-                mrb_float f = mrb_float(*sp);
+                mrb_float f = mrb_float(ARGV[arg_i]);
 
                 if (!FIXABLE(f)) {
                   mrb_raise(mrb, E_RANGE_ERROR, "float too big for int");
@@ -696,10 +703,10 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
               mrb_raise(mrb, E_TYPE_ERROR, "no implicit conversion of String into Integer");
               break;
             default:
-              *p = mrb_fixnum(mrb_Integer(mrb, *sp));
+              *p = mrb_fixnum(mrb_Integer(mrb, ARGV[arg_i]));
               break;
           }
-          sp++;
+          arg_i++;
           i++;
         }
       }
@@ -709,7 +716,7 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         mrb_bool *boolp = va_arg(ap, mrb_bool*);
 
         if (i < argc) {
-          mrb_value b = *sp++;
+          mrb_value b = ARGV[arg_i++];
           *boolp = mrb_test(b);
           i++;
         }
@@ -723,7 +730,7 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         if (i < argc) {
           mrb_value ss;
 
-          ss = *sp++;
+          ss = ARGV[arg_i++];
           *symp = to_sym(mrb, ss);
           i++;
         }
@@ -738,14 +745,14 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         type = va_arg(ap, struct mrb_data_type const*);
         if (*format == '!') {
           format++;
-          if (i < argc && mrb_nil_p(*sp)) {
+          if (i < argc && mrb_nil_p(ARGV[arg_i])) {
             *datap = 0;
-            i++; sp++;
+            i++; arg_i++;
             break;
           }
         }
         if (i < argc) {
-          *datap = mrb_data_get_ptr(mrb, *sp++, type);
+          *datap = mrb_data_get_ptr(mrb, ARGV[arg_i++], type);
           ++i;
         }
       }
@@ -787,10 +794,10 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         if (argc > i) {
           *pl = argc-i;
           if (*pl > 0) {
-            *var = sp;
+            *var = ARGV + arg_i;
           }
           i = argc;
-          sp += *pl;
+          arg_i += *pl;
         }
         else {
           *pl = 0;
@@ -803,6 +810,9 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
       break;
     }
   }
+
+#undef ARGV
+
   if (!c && argc > i) {
     mrb_raise(mrb, E_ARGUMENT_ERROR, "wrong number of arguments");
   }