stealth_check_mapping_for_destruct can fail in the gc

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=904

Reported by Martin Stjernholm mast@roxen.com

The following program causes a fatal in the gc.

int main()
{
  object o = class {}();
  mapping m = ([o: ({17})]);
  destruct (o);
  gc();
}