avformat/vobsub: compare correct packet stream IDs

The stream ID is essentially an arbitrary number defined by the .idx
file headers. They have to match the IDs in the .sub stream. The vobsub
demuxer assumed the IDs would just start from 0, increassing by 1 for
each stream. This is not correct. In the sample I had, the IDs were
starting from 1, leading to no subtitles being displayed at all.

Fix this by using the correct stream ID.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a47ad06b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mpeg.c

@@ -939,7 +939,7 @@ static int vobsub_read_packet(AVFormatContext *s, AVPacket *pkt)
         total_read += pkt_size;
 
         /* the current chunk doesn't match the stream index (unlikely) */
-        if ((startcode & 0x1f) != idx_pkt.stream_index)
+        if ((startcode & 0x1f) != s->streams[idx_pkt.stream_index]->id)
             break;
 
         ret = av_grow_packet(pkt, to_read);