Bug 1199901 - Bustage fix due to rebase. a=bustage

Based on patch here https://reviewboard.mozilla.org/r/17787/diff/1/
CLOSED TREE

--HG--
extra : commitid : 6G7NhTioaaP

dom/notification/Notification.cpp

@@ -1006,8 +1006,10 @@ protected:
     AssertIsOnMainThread();
 
     MOZ_ASSERT(mNotificationRef);
-    notification->mObserver = nullptr;
-
+    Notification* notification = mNotificationRef->GetNotification();
+    if (notification) {
+      notification->mObserver = nullptr;
+    }
   }
 };