mpegts: use ffio_ensure_seekback()

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

libavformat/mpegts.c

@@ -2003,6 +2003,8 @@ static int mpegts_read_header(AVFormatContext *s)
     int len;
     int64_t pos;
 
+    ffio_ensure_seekback(pb, s->probesize);
+
     /* read the first 8192 bytes to get packet size */
     pos = avio_tell(pb);
     len = avio_read(pb, buf, sizeof(buf));