Bug 1409249: Follow-up: Fix libjar ref counting bustage. r=bustage

MozReview-Commit-ID: IbazBuzLGvN

--HG--
extra : rebase_source : a8f9c3562868b190ffa336cb306903c5608a4f5f

modules/libjar/nsJARProtocolHandler.cpp

@@ -73,6 +73,8 @@ nsJARProtocolHandler::GetSingleton()
             gJarHandler = nullptr;
             return nullptr;
         }
+        // We release this reference on module shutdown.
+        NS_ADDREF(gJarHandler);
         return jar.forget();
     }
     return do_AddRef(gJarHandler);