Fix crash in tst_qtreewidget.

A division by zero occurred since an empty model was used.

Task-number: QTBUG-28611

Change-Id: I1f9e869bda9f76a1c97840dff6ba48ab813bce9a
Reviewed-by: Janne Anttila <janne.anttila@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>

src/plugins/accessible/widgets/itemviews.cpp

@@ -489,7 +489,7 @@ int QAccessibleTree::childCount() const
 
 QAccessibleInterface *QAccessibleTree::child(int index) const
 {
-    if ((index < 0) || (!view()->model()))
+    if (index < 0 || !view()->model() || !view()->model()->columnCount())
         return 0;
     int hHeader = horizontalHeader() ? 1 : 0;