finish MessageRecipientAsyncTask when context is destroyed

Closes #3591

// FREEBIE

src/org/thoughtcrime/securesms/MessageDetailsActivity.java

@@ -256,6 +256,7 @@ public class MessageDetailsActivity extends PassphraseRequiredActionBarActivity
       Context context = getContext();
       if (context == null) {
         Log.w(TAG, "associated context is destroyed, finishing early");
+        return null;
       }
 
       Recipients recipients;