Clear unused stack region that may refer freed objects; fix #3596
add_gray_list(mrb_state *mrb, mrb_gc *gc, struct RBasic *obj)
{
#ifdef MRB_GC_STRESS
  if (obj->tt > MRB_TT_MAXDEFINE) {
    abort();
  }
#endif
  paint_gray(obj);
  obj->gcnext = gc->gray_list;
  gc->gray_list = obj;
}

add_gray_list(mrb_state *mrb, mrb_gc *gc, struct RBasic *obj)
{
#ifdef MRB_GC_STRESS
  if (obj->tt > MRB_TT_MAXDEFINE) {
    abort();
  }
#endif
  paint_gray(obj);
  obj->gcnext = gc->gray_list;
  gc->gray_list = obj;
}
