Clean up a partially scrolled QRollEffect widget

If the associated widget pointer got zeroed while QRollEffect was
scrolling, the partially scrolled QRollEffect widget remained on
screen indefinitely as 'done' was never set to true.

Change-Id: If1567ea740e81b5501137d10db471ca97d295ed8
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
(cherry picked from qtbase/486601fbc4a7e9eaeb5df5bb3966c3aec31d8d65)
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>