Item views: prevent negative cache buffer

A negative cache buffer does not make much sense, and the item views
would go crazy and start creating/destructing delegates endlessly.

Task-number: QTBUG-38725
Change-Id: I1fbba1f3130a99af67fbc4c2aba4d3199d0554a9
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>