Properly clear out old state when receiving a new intent.

This fixes the following scenario:
- Get a mail notification
- Click on notification -> message view appears
- Press home
- Get a new notification
- Click on notification -> last instead of new message is displayed

src/com/fsck/k9/activity/MessageList.java

@@ -236,6 +236,10 @@ public class MessageList extends K9FragmentActivity implements MessageListFragme
         removeMessageListFragment();
         removeMessageViewFragment();
 
+        mMessageReference = null;
+        mSearch = null;
+        mFolderName = null;
+
         decodeExtras(intent);
         initializeDisplayMode(null);
         initializeFragments();