Clear unused stack region that may refer freed objects; fix #3596
link_free_heap_page(mrb_gc *gc, mrb_heap_page *page)
{
  page->free_next = gc->free_heaps;
  if (gc->free_heaps) {
    gc->free_heaps->free_prev = page;
  }
  gc->free_heaps = page;
}

link_free_heap_page(mrb_gc *gc, mrb_heap_page *page)
{
  page->free_next = gc->free_heaps;
  if (gc->free_heaps) {
    gc->free_heaps->free_prev = page;
  }
  gc->free_heaps = page;
}
