Merge pull request #1313 from mimaki/fix-mrb_get_args-z-format

ArgumentError raised when mrb_get_args(mrb, "z", ...) receives substring.

src/class.c

@@ -487,7 +487,7 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         if (i < argc) {
           ss = to_str(mrb, *sp++);
           s = mrb_str_ptr(ss);
-          if (strlen(s->ptr) != s->len) {
+          if (strlen(s->ptr) < s->len) {
             mrb_raise(mrb, E_ARGUMENT_ERROR, "String contains NUL");
           }
           *ps = s->ptr;