mxfdec: minor simplification.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavformat/mxfdec.c

@@ -2625,7 +2625,7 @@ static int mxf_read_packet_old(AVFormatContext *s, AVPacket *pkt)
             pkt->stream_index = index;
             pkt->pos = klv.offset;
 
-            codec = s->streams[index]->codec;
+            codec = st->codec;
 
             if (codec->codec_type == AVMEDIA_TYPE_VIDEO && next_ofs >= 0) {
                 /* mxf->current_edit_unit good - see if we have an