avformat/mpegts: fix resync seek

The seek ended up seeking before the begin, which caused problems
Fixes initial sync issues with libbluray
Fixes Ticket3117

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

libavformat/mpegts.c

@@ -1993,7 +1993,9 @@ static int read_packet(AVFormatContext *s, uint8_t *buf, int raw_packet_size, co
         /* check packet sync byte */
         if ((*data)[0] != 0x47) {
             /* find a new packet start */
-            avio_seek(pb, -raw_packet_size, SEEK_CUR);
+            uint64_t pos = avio_tell(pb);
+            avio_seek(pb, -FFMIN(raw_packet_size, pos), SEEK_CUR);
+
             if (mpegts_resync(s) < 0)
                 return AVERROR(EAGAIN);
             else