mpegts: Silence "can't seek" warning on unseekable

Do not try to seek when we already know we are not allowed to.
Silences warning that always happens when streaming.

Signed-off-by: Zohar Kelrich <lumimies@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/mpegts.c

@@ -1488,7 +1488,7 @@ static int mpegts_read_header(AVFormatContext *s,
         /* normal demux */
 
         /* first do a scaning to get all the services */
-        if (avio_seek(pb, pos, SEEK_SET) < 0)
+        if (pb->seekable && avio_seek(pb, pos, SEEK_SET) < 0)
             av_log(s, AV_LOG_ERROR, "Unable to seek back to the start\n");
 
         mpegts_open_section_filter(ts, SDT_PID, sdt_cb, ts, 1);