Fix hover event crash

It was possible for a hover event to be sent to a QQuickItem that has
already been scheduled to be deleted (through deleteLater()). This
lead to an access on an already-freed object when the leave event
is generated. This change ensures that the item is part of a scene
before generating the hover enter event.

Task-number: QTBUG-32771

Change-Id: I69adb6bbd0ae52c70a6bda4e6c918b7671549a4c