Clear unused stack region that may refer freed objects; fix #3596
prepare_incremental_sweep(mrb_state *mrb, mrb_gc *gc)
{
  gc->state = MRB_GC_STATE_SWEEP;
  gc->sweeps = gc->heaps;
  gc->live_after_mark = gc->live;
}

prepare_incremental_sweep(mrb_state *mrb, mrb_gc *gc)
{
  gc->state = MRB_GC_STATE_SWEEP;
  gc->sweeps = gc->heaps;
  gc->live_after_mark = gc->live;
}
