h264_sei: check SEI size

Signed-off-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
h264: prevent theoretical infinite loop in SEI parsing

Properly address CVE-2011-3946 and parse bitstream as described in the spec.

CC: libav-stable@libav.org
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind