avsdec: Set dimensions instead of relying on the demuxer.

This fixes out of array writes.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avs.c

@@ -162,6 +162,7 @@ static av_cold int avs_decode_init(AVCodecContext * avctx)
     AvsContext *const avs = avctx->priv_data;
     avctx->pix_fmt = PIX_FMT_PAL8;
     avcodec_get_frame_defaults(&avs->picture);
+    avcodec_set_dimensions(avctx, 318, 198);
     return 0;
 }