ZWR: extra thread safety in level 3 hack
ZWR: extra thread safety in level 3 hack

git-svn-id: 13244e1f4c8acb548f3f909f1ae0d151defad92d@208 1ea6ab17-982b-0410-b0ab-91172c90a6e5

mikeash/MAZeroingWeakRef,mikeash/MAZeroingWeakRef