Fix ref<T>::~ref to not try to RemoveRef if already deleted.
-rw-r--r-- 290 Makefile
-rw-r--r-- 1324 README.md
-rw-r--r-- 1432 test.cc
-rw-r--r-- 793 zcpointer.cc
-rw-r--r-- 2920 zcpointer.h