avformat/oggdec: dont read timestamps from EOS pages of ogm videos

Some muxers store invalid timestamps there, which breaks seeking
Fixes Ticket2739
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/oggdec.c

@@ -808,6 +808,11 @@ static int64_t ogg_read_timestamp(AVFormatContext *s, int stream_index,
            && !ogg_packet(s, &i, &pstart, &psize, pos_arg)) {
         if (i == stream_index) {
             struct ogg_stream *os = ogg->streams + stream_index;
+            // Dont trust the last timestamps of a ogm video
+            if (    (os->flags & OGG_FLAG_EOS)
+                && !(os->flags & OGG_FLAG_BOS)
+                && os->codec == &ff_ogm_video_codec)
+                continue;
             pts = ogg_calc_pts(s, i, NULL);
             ogg_validate_keyframe(s, i, pstart, psize);
             if (os->pflags & AV_PKT_FLAG_KEY) {