Don't crash with deferred properties

There are cases where a qmlExecuteDeferred(o) can be postponed until the
context of o is being destroyed, at which point it's too late to create
an object in that context.

Task-number: QTBUG-33112
Change-Id: I7f981b5e34e3cb8a52c00de4742a7242d7e4df54
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>