mpegts: Warn if unable to seek back

Originally committed as revision 24477 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mpegts.c

@@ -1387,7 +1387,8 @@ static int mpegts_read_header(AVFormatContext *s,
         /* normal demux */
 
         /* first do a scaning to get all the services */
-        url_fseek(pb, pos, SEEK_SET);
+        if (url_fseek(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);