Fix UB in QGraphicsScene::wheelEvent()

operator--() would iterate before begin() if the
list was empty. This is UB, and will crash in Qt 6,
where begin()/end() can return an iterator pointing
to a nullptr if the list is empty.

Change-Id: I39c3a8ebb09fcad75d42019b02426ac5ac05eed9
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/widgets/graphicsview/qgraphicsscene.cpp

@@ -4101,7 +4101,8 @@ void QGraphicsScene::wheelEvent(QGraphicsSceneWheelEvent *wheelEvent)
     // Remove all popups after the one found, or all or them if no popup is under the mouse.
     // Then continue with the event.
     QList<QGraphicsWidget *>::const_iterator iter = d->popupWidgets.constEnd();
-    while (--iter >= d->popupWidgets.constBegin() && !wheelCandidates.isEmpty()) {
+    while (iter > d->popupWidgets.constBegin() && !wheelCandidates.isEmpty()) {
+        --iter;
         if (wheelCandidates.first() == *iter || (*iter)->isAncestorOf(wheelCandidates.first()))
             break;
         d->removePopup(*iter);