vc1: Use codec ID from AVCodecContext while parsing frame header

This fixes a segfault with samples that I have (both of them MPEG-TS). Looks like
avctx->codec is not being set during parsing.

Signed-off-by: Kostya Shishkov <kostya.shishkov@gmail.com>

libavcodec/vc1.c

@@ -576,7 +576,7 @@ int ff_vc1_parse_frame_header(VC1Context *v, GetBitContext* gb)
 
     if (v->finterpflag)
         v->interpfrm = get_bits1(gb);
-    if (v->s.avctx->codec->id == AV_CODEC_ID_MSS2)
+    if (v->s.avctx->codec_id == AV_CODEC_ID_MSS2)
         v->respic   =
         v->rangered =
         v->multires = get_bits(gb, 2) == 1;