Fixed another assert in qwindowskeymapper

When we get a WM_CHAR message we do not need to add anything to the key
map as these messages already contain the character code in question.

Task-number: QTBUG-35532

Change-Id: If73993cd873b7c616876b9ba52ceba6b27a6a7b1
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
(cherry picked from commit 8e2f95e5a2ebc54c600d1db416b1dd84e433d115)