Fix another type of crash when dragging QDockWidget in and out

QDockWidgetPrivate::mouseMoveEvent calls q->move() which can result
in a call to QDockWidgetPrivate::endDrag(), which will delete the
state struct.  Therefore mouseMoveEvent needs to check whether it's
null or not before accessing it.

Task-number: QTBUG-31672
Change-Id: I91a31620f16a80b31b65b2742e7937ae8960f0fd
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>