QStackedLayout: Fix UB (invalid cast) in qt_wasDeleted()

Fixup of commit b4995eb749.
We can't call QWidgetPrivate::get(w) on a deleted QWidget, because of
the call to the member function QWidget::d_func. We can however call
QObjectPrivate::get since we still are in the QObject destructor.

tst_qstackedlayout now pass without ubsan Warnings.

Change-Id: I4e839a97ddbd1cf21435a8fca76523b98a1f7d9b
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/kernel/qobject_p.h

@@ -198,6 +198,7 @@ public:
     static QObjectPrivate *get(QObject *o) {
         return o->d_func();
     }
+    static const QObjectPrivate *get(const QObject *o) { return o->d_func(); }
 
     int signalIndex(const char *signalName, const QMetaObject **meta = 0) const;
     inline bool isSignalConnected(uint signalIdx, bool checkDeclarative = true) const;

src/widgets/kernel/qstackedlayout.cpp

@@ -253,7 +253,7 @@ QLayoutItem *QStackedLayout::itemAt(int index) const
 // on the object then)
 static bool qt_wasDeleted(const QWidget *w)
 {
-    return QWidgetPrivate::get(w)->wasDeleted;
+    return QObjectPrivate::get(w)->wasDeleted;
 }