huffyuvdec: check width size for yuv422p

Avoid out of array accesses.

CC: libav-stable@libav.org
Bug-Id: CVE-2013-0848
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit a7153444df9040bf6ae103e0bbf6104b66f974cb)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

Conflicts:
	libavcodec/huffyuvdec.c