QWidgetWindow: Guard reference to own widget

~QWidgetPrivate() may end up calling QWidgetPrivate::
deleteTLSysExtra() which, in turn, calls QWindow::destroy().
This sends an event to the window itself. This reaches
QWidgetWindow::event() which will forward the event to
the widget. However, the widget has just been deleted
since the sequence was initiated by ~QObject().

Task-number: QTBUG-53103
Change-Id: Ib511714a76bbc1e734d6f2800a983eb1459bbf0b
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>