Fix crash on QV8Engine destruction

QObjects tracked by QV8Engine can end up causing bindings to execute
when the QV8Engine is being deleted, which can call through to
QV8Engine::fromVariant(). This change ensure that that is valid
while the QObjects are being destroyed.

Task-number: QTBUG-31555

Change-Id: Iaea402ff65993bb2322af403d3590653005e30fe
Reviewed-by: Alan Alpert <aalpert@blackberry.com>