Utils: Do not crash when clicking in empty line of HistoryCompleter

Removing lines from a HistoryCompleter can result in a crash if the
mouse click happens on an empty line.

Change-Id: I02ee7c4705e1a4bc6b63ccdae207b5078b03360e
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/historycompleter.cpp

@@ -117,8 +117,11 @@ private:
             if (layoutDirection() == Qt::LeftToRight)
                 rr = viewport()->width() - event->x();
             if (rr < clearButtonSize.width()) {
-                model->removeRow(indexAt(event->pos()).row());
-                return;
+                const QModelIndex index = indexAt(event->pos());
+                if (index.isValid()) {
+                    model->removeRow(indexAt(event->pos()).row());
+                    return;
+                }
             }
         }
         QListView::mousePressEvent(event);