Fixes issue 6703

- onPrepareActionMode must be called before computeBatchDirection
  because computeBatchDirection ends up referencing mMarkAsRead /
  mMarkAsUnread and mFlag / mUnflag which could be null otherwise.

src/com/fsck/k9/fragment/MessageListFragment.java

@@ -2292,12 +2292,12 @@ public class MessageListFragment extends Fragment implements OnItemClickListener
             mSelectedCount += selectedCountDelta;
         }
 
-        computeBatchDirection();
-        updateActionModeTitle();
-
         // make sure the onPrepareActionMode is called
         mActionMode.invalidate();
 
+        computeBatchDirection();
+        updateActionModeTitle();
+
         computeSelectAllVisibility();
 
         mAdapter.notifyDataSetChanged();