Clear unused stack region that may refer freed objects; fix #3596
mrb_gc_destroy(mrb_state *mrb, mrb_gc *gc)
{
  free_heap(mrb, gc);
#ifndef MRB_GC_FIXED_ARENA
  mrb_free(mrb, gc->arena);
#endif
}

mrb_gc_destroy(mrb_state *mrb, mrb_gc *gc)
{
  free_heap(mrb, gc);
#ifndef MRB_GC_FIXED_ARENA
  mrb_free(mrb, gc->arena);
#endif
}
