Fix crash when loading invalid QML with behavior on invalid group property

Fix crash when loading invalid QML with behavior on invalid group property

Behaviors require the creation of a meta-object. However when trying to
create a behavior on a non-existent group property, we don't have a
base meta-object to base the "new" meta-object on, therefore this patch
adds a null pointer check.

The error in the QML file itself will be caught later on. The added test
ensures that as well as that it doesn't crash of course.

Change-Id: If73116053464e7e69b02ef59e8387060835083c8
Task-number: QTBUG-40369
Reviewed-by: Sérgio Martins <sergio.martins@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>