Fix UB in QWidgetItemV2::~QWidgetItemV2

We might get there because 'wid' is already partially deleted (called
from ~QObject). In that case, it is an undefined behavior to call a
QWidget member function on it. Use QObjectPrivate::get instead.

Change-Id: I6da314bf8385684d1332aa031a2d92012941303b
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/widgets/kernel/qlayoutitem.cpp

@@ -767,7 +767,7 @@ QWidgetItemV2::QWidgetItemV2(QWidget *widget)
 QWidgetItemV2::~QWidgetItemV2()
 {
     if (wid) {
-        QWidgetPrivate *wd = wid->d_func();
+        auto *wd = static_cast<QWidgetPrivate *>(QObjectPrivate::get(wid));
         if (wd->widgetItem == this)
             wd->widgetItem = 0;
     }