bug 1502841 - follow-up to ensure reentering DataStorageSharedThread::Shutdown() doesn't cause problems r=jcj

We shouldn't reenter DataStorageSharedThread::Shutdown(), but it may be
possible. To guard against potentially attempting to shut down the shared thread
more than once, we can check gDataStorageSharedThreadShutDown first.

Differential Revision: https://phabricator.services.mozilla.com/D12050

--HG--
extra : moz-landing-system : lando

security/manager/ssl/DataStorage.cpp

@@ -110,7 +110,7 @@ DataStorageSharedThread::Shutdown()
   MOZ_ASSERT(XRE_IsParentProcess());
   StaticMutexAutoLock lock(sDataStorageSharedThreadMutex);
 
-  if (!gDataStorageSharedThread) {
+  if (!gDataStorageSharedThread || gDataStorageSharedThreadShutDown) {
     return NS_OK;
   }