Don't dereference null QQuickWindowPrivate.

windowDestroyed() triggers a sendPostedEvent(DeferredDelete) to
cleanup GL resources. If the window was deleted through deleteLater()
it will also be deleted at this point.

Change-Id: I3b0cc0f36c4a543c5fbaa4af107dc4aa0c8b6725
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>