Avoid crashing when applying CSS to disconnected scrollbars

Assume the scrollbar has no scroll corner if it isn't attached to
any scrollable area.

Task-number: QTBUG-37679
Change-Id: Ib1bd2a4b1aa504885d52932b6c0fbd76daa356a5
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>