NULL-check the entry in NonAllocatingMap before setting on it.

Using just an assert will still cause a crash in a release build.

BUG=http://code.google.com/p/chromium/issues/detail?id=238757
R=mark@chromium.org

Review URL: https://breakpad.appspot.com/593003

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1174 4c0a9323-5329-0410-9bdc-e9ce6186880e