set list adapter to null when appropriate

set list adapter to null when (recipients or threadId == null) to
prevent old messages from being displayed when forwarding a
message to a recipient for which there is no existing
 conversation.

Fixes #2883
Closes #2884

// FREEBIE

src/org/thoughtcrime/securesms/ConversationFragment.java

@@ -103,6 +103,10 @@ public class ConversationFragment extends ListFragment
 
     initializeResources();
     initializeListAdapter();
+
+    if (threadId == -1) {
+      getLoaderManager().restartLoader(0, null, this);
+    }
   }
 
   private void initializeResources() {