Null check for window on pointer_axis

The window is already destroyed and server doesn't get a message yet.

Change-Id: Ia9dd0ce891a712d1aa913fcb3313a5fe300788ab
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>