mmvideo: check horizontal coordinate too

Fixes out of array accesses.

Bug-Id: CVE-2013-3672
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>
