fix pointer dereference after realloc

In src/vm.c: mrb_funcall_with_block
stack_extend may realloc mrb->c->stbase, if argv points on mruby's stack,
then it points invalid address after stack_extend.

e.g. src/class.c: mrb_instance_new

This code:

```ruby
class A
  def initialize(a0,a1,a2,a3,a4)
    a0.is_a? Array
  end
end

def f(a0,a1,a2,a3,a4)
  A.new(a0,a1,a2,a3,a4)
  f(a0,a1,a2,a3,a4)
end

f(0,1,2,3,4)
```

is expected to get exception
```
stack level too deep. (limit=(0x40000 - 128)) (SystemStackError)
```

but get segfault.
Signed-off-by: Go Saito <gos@iij.ad.jp>

src/vm.c

@@ -340,6 +340,7 @@ mrb_funcall_with_block(mrb_state *mrb, mrb_value self, mrb_sym mid, mrb_int argc
     mrb_sym undef = 0;
     mrb_callinfo *ci;
     int n;
+    ptrdiff_t voff = -1;
 
     if (!mrb->c->stack) {
       stack_init(mrb);
@@ -363,6 +364,9 @@ mrb_funcall_with_block(mrb_state *mrb, mrb_value self, mrb_sym mid, mrb_int argc
     ci->argc = argc;
     ci->target_class = c;
     mrb->c->stack = mrb->c->stack + n;
+    if (mrb->c->stbase <= argv && argv < mrb->c->stend) {
+      voff = argv - mrb->c->stbase;
+    }
     if (MRB_PROC_CFUNC_P(p)) {
       ci->nregs = argc + 2;
       stack_extend(mrb, ci->nregs, 0);
@@ -371,6 +375,9 @@ mrb_funcall_with_block(mrb_state *mrb, mrb_value self, mrb_sym mid, mrb_int argc
       ci->nregs = p->body.irep->nregs + n;
       stack_extend(mrb, ci->nregs, argc+2);
     }
+    if (voff >= 0) {
+      argv = mrb->c->stbase + voff;
+    }
     mrb->c->stack[0] = self;
     if (undef) {
       mrb->c->stack[1] = mrb_symbol_value(undef);