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();
}