Fix the assert failure in QShortcutMap::find

Don't send Qt::Key_unknown down the shortcut handling pipe, because
unknown keys are not part of a shortcut sequence and results in an assert.

Task-number: QTBUG-30871
Change-Id: Idd451fb793bd07124a96d75ed7b936f7ff584f28
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
(cherry picked from qtbase/c5a6b894e7484544c44a8a415a84e196c4b633c3)