Bug 767406 - don't assume the tables are initialized in StartupCache::WriteToDisk; r=mwu

startupcache/StartupCache.cpp

@@ -414,7 +414,7 @@ StartupCache::WriteToDisk()
   nsresult rv;
   mStartupWriteInitiated = true;
 
-  if (mTable.Count() == 0)
+  if (!mTable.IsInitialized() || mTable.Count() == 0)
     return;
 
   nsCOMPtr<nsIZipWriter> zipW = do_CreateInstance("@mozilla.org/zipwriter;1");