QGraphicsWidget: don't dereference nullptr

Unlikely that one item is nullptr and the other item doesn't have a
scene, but we do already test for the scene pointer, so don't continue
if it's nullptr.

Fixes static analyzer warning f59576ecf6618447c4f9c7be93fc737f

Pick-to: 6.1
Change-Id: I6d436bb1211ddd412821d6978bab25192033f5e5
Reviewed-by: David Skoland <david.skoland@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/graphicsview/qgraphicswidget.cpp

@@ -2143,7 +2143,7 @@ void QGraphicsWidget::setTabOrder(QGraphicsWidget *first, QGraphicsWidget *secon
         return;
     }
     QGraphicsScene *scene = first ? first->scene() : second->scene();
-    if (!scene && (!first || !second)) {
+    if (!scene) {
         qWarning("QGraphicsWidget::setTabOrder: assigning tab order from/to the"
                  " scene requires the item to be in a scene.");
         return;