Fix race condition in testMultiple()

* If we close the activity while a requeryList() is pending in the
  handler queue, we'll eventually try to operate on a closed cursor
  when we get to testMultiple().
* The fix: Return immediately if the cursor is not available.

Fixes bug http://b/2180416
Followup for bug http://b/2149083

src/com/android/email/activity/MessageList.java

@@ -752,6 +752,9 @@ public class MessageList extends ListActivity implements OnItemClickListener, On
      */
     private boolean testMultiple(Set<Long> selectedSet, int column_id, boolean defaultflag) {
         Cursor c = mListAdapter.getCursor();
+        if (c == null || c.isClosed()) {
+            return false;
+        }
         c.moveToPosition(-1);
         while (c.moveToNext()) {
             long id = c.getInt(MessageListAdapter.COLUMN_ID);