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>

src/widgets/widgets/qdockwidget.cpp

@@ -873,7 +873,7 @@ bool QDockWidgetPrivate::mouseMoveEvent(QMouseEvent *event)
         QPoint pos = event->globalPos() - state->pressPos;
         q->move(pos);
 
-        if (!state->ctrlDrag)
+        if (state && !state->ctrlDrag)
             mwlayout->hover(state->widgetItem, event->globalPos());
 
         ret = true;