mark_context_stack(mrb_state *mrb, struct mrb_context *c)
{
  size_t i;
  size_t e;

  if (c->stack == NULL) return;
  e = c->stack - c->stbase;
  if (c->ci) e += c->ci->nregs;
  if (c->stbase + e > c->stend) e = c->stend - c->stbase;
  for (i=0; i<e; i++) {
    mrb_value v = c->stbase[i];

    if (!mrb_immediate_p(v)) {
      if (mrb_basic_ptr(v)->tt == MRB_TT_FREE) {
        c->stbase[i] = mrb_nil_value();
      }
      else {
        mrb_gc_mark(mrb, mrb_basic_ptr(v));
      }
    }
  }
}
Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.