mpegts: use seek_back() for all seek backs

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mpegts.c

@@ -2080,7 +2080,7 @@ static int mpegts_read_header(AVFormatContext *s)
                 st->start_time / 1000000.0, pcrs[0] / 27e6, ts->pcr_incr);
     }
 
-    avio_seek(pb, pos, SEEK_SET);
+    seek_back(s, pb, pos);
     return 0;
  fail:
     return -1;