[qtdeclarative] Don't crash due to KeyRelease on an unloaded Item.

Applying upstream change:

commit cb9672c86d2679bcb0d9e10f76b12eb03f4c515e
Author: Shawn Rutledge <shawn.rutledge@digia.com>
Date:   Tue Aug 27 13:36:36 2013 +0200

Task-number: QTBUG-33127