Crash when canceling gesture without target in scene

When a gesture "G" is triggered by events sent to QGV->viewport() (for example, QMouseEvent), and that trigger further causes cancelling of other gestures via cancel policy (QGesture::CancelAllInContext), and there is no recipient for the gesture "G" under the hotspot, Qt will assert/crash.

The crash is in QGraphicsScenePrivate::cancelGesturesForChildren, caused by null originalItem (Q_ASSERTed in debug builds).

The gesture "G" should be delivered to the scene, and if there is no recipient for the gesture, no cancels should happen. I.e. remove assertion and return if originalItem is null.

Test case attached.