avsdec: Set dimensions instead of relying on the demuxer.

The decode function assumes that the video will have those dimensions.

Fixes CVE-2012-2801

CC:libav-stable@libav.org

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Anton Khirnov <anton@khirnov.net>