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>
(cherry picked from commit 85f477935c)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavcodec/avs.c

@@ -158,6 +158,7 @@ avs_decode_frame(AVCodecContext * avctx,
 static av_cold int avs_decode_init(AVCodecContext * avctx)
 {
     avctx->pix_fmt = PIX_FMT_PAL8;
+    avcodec_set_dimensions(avctx, 318, 198);
     return 0;
 }