OSX: avoid crash in invalidateBuffer_resizeHelper on parentless widget

KDE4 has a KMenu::addTitle function that adds a distinctive item to
a menu, as the predecessor to Qt5's QMenu::addSection. On OS X this
function leads to a null pointer access because
invalidateBuffer_resizeHelper is called on a parentless widget.
This patch filters out such widgets.

Task-number: QTBUG-41716
Change-Id: I8f62a14d201ce44e038ceff87ee5db98277f1b87
Reviewed-by: Morten Johan Sørvig <[email protected]>

Author: RJVB

src/gui/kernel/qwidget_mac.mm

@@ -4619,7 +4619,7 @@ const QRect frameRect(QPoint(x - fStrut.left(), y - fStrut.top()),
 
         setWSGeometry(false, oldRect);
 
-        if (isResize && QApplicationPrivate::graphicsSystem())
+        if (isResize && q->parentWidget() && QApplicationPrivate::graphicsSystem())
             invalidateBuffer_resizeHelper(oldp, olds);
     }