Win: Fixed assert in QKeyMapperPrivate::possibleKeys

Trying to detect key combinations for events that have an invalid
virtual key will fail in any case and does not make sense.

Task-number: QTBUG-36061
Change-Id: Ia30cb7bf6c7bae06eb84c23b356a53a2eb6e86aa
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>