QAbstractItemView: do not access invalid model indices (2/N)

Similar to the parent patch, the private selectAll() was doing
two out of bounds accesses on an empty model. Guard it.

Change-Id: If0f3ce1e6c44a152791313e47db79985e71ef955
Pick-to: 5.15 6.2 6.3
Reviewed-by: David Faure <david.faure@kdab.com>

src/widgets/itemviews/qabstractitemview.cpp

@@ -4623,6 +4623,8 @@ void QAbstractItemViewPrivate::selectAll(QItemSelectionModel::SelectionFlags com
 {
     if (!selectionModel)
         return;
+    if (!model->hasChildren(root))
+        return;
 
     QItemSelection selection;
     QModelIndex tl = model->index(0, 0, root);