Do not show warning on null result in getProcAddress

Other platforms do not do this either. What is more, some of the
extension handling code will try to resolve functions that are not
necessiarly present, and this is not an error.

Change-Id: I39dad8f8d89fc45de3ea83f04727a8e38b6a3387
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>