[XCB] Fix crash in QML drag-and-drop

Use QPointer to make sure that we are not trying to destroy already
deleted object.

Task-number: QTBUG-35702

Change-Id: Ib746996787488e636f25e6ea5be0571607ee2ded
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/platforms/xcb/qxcbdrag.cpp

@@ -1044,7 +1044,8 @@ void QXcbDrag::timerEvent(QTimerEvent* e)
                  - showing dialog box on drop event where user's response takes more time than XdndDropTransactionTimeout (QTBUG-14493)
                  - dnd takes unusually long time to process data
                  */
-                t.drag->deleteLater();
+                if (t.drag)
+                    t.drag->deleteLater();
                 transactions.removeAt(i--);
             } else {
                 stopTimer = false;

src/plugins/platforms/xcb/qxcbdrag.h

@@ -157,7 +157,7 @@ private:
         xcb_window_t proxy_target;
         QWindow *targetWindow;
 //        QWidget *embedding_widget;
-        QDrag *drag;
+        QPointer<QDrag> drag;
         QTime time;
     };
     QList<Transaction> transactions;