Don't notify on MMS delivery failure if message has been deleted.

// FREEBIE

Fixes #2453

src/org/thoughtcrime/securesms/jobs/MmsSendJob.java

@@ -244,8 +244,10 @@ public class MmsSendJob extends SendJob {
     long       threadId   = DatabaseFactory.getMmsDatabase(context).getThreadIdForMessage(messageId);
     Recipients recipients = DatabaseFactory.getThreadDatabase(context).getRecipientsForThreadId(threadId);
 
+    if (recipients != null) {
       MessageNotifier.notifyMessageDeliveryFailed(context, recipients, threadId);
     }
+  }