the problem is that nsZipArchive and zlib are not thread safe.
for now, add a lock to prevent multiple threads from getting into that code
r=warren,dveditz
super-end-of-game-review=mscott
git-svn-id: svn://10.0.0.236/trunk@79367 18797224-902f-48f8-a5cc-f745e15eee43