QAbstractItemView: Fix UB (invalid downcast)

Just use QWidgetPrivate::get() instead.

Fixes UBSan error:

  qabstractitemview.cpp:3814:61: runtime error: downcast of address 0x2b859001aa70 which does not point to an object of type 'QAbstractItemView'
    0x2b859001aa70: note: object is of type 'QWidget'

Change-Id: I0460fd8a0681e122d440755ebf07018d273b93f8
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/widgets/itemviews/qabstractitemview.cpp

@@ -3821,7 +3821,7 @@ void QAbstractItemView::doAutoScroll()
     int horizontalValue = horizontalScroll->value();
 
     QPoint pos = d->viewport->mapFromGlobal(QCursor::pos());
-    QRect area = static_cast<QAbstractItemView*>(d->viewport)->d_func()->clipRect(); // access QWidget private by bending C++ rules
+    QRect area = QWidgetPrivate::get(d->viewport)->clipRect();
 
     // do the scrolling if we are in the scroll margins
     if (pos.y() - area.top() < margin)