Fix null-pointer access in QQuickVisualDataModelPrivate

I observed null cachItem->contextData which lead to null-pointer access
on cacheItem->contextData->destroy().

Task-number: QTBUG-31439

Change-Id: I91f28a3ee1ac83446ecde1801a1cb7962fb883f3
Reviewed-by: Alan Alpert <aalpert@blackberry.com>