avformat/mpegts: use actually read packet size in mpegts_resync special case

Fixes: infinite loop
Fixes: 37986/clusterfuzz-testcase-minimized-ffmpeg_dem_MPEGTSRAW_fuzzer-5292311517462528 -

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mpegts.c

@@ -2875,8 +2875,8 @@ static int mpegts_resync(AVFormatContext *s, int seekback, const uint8_t *curren
     int64_t back = FFMIN(seekback, pos);
 
     //Special case for files like 01c56b0dc1.ts
-    if (current_packet[0] == 0x80 && current_packet[12] == 0x47) {
-        avio_seek(pb, 12 - back, SEEK_CUR);
+    if (current_packet[0] == 0x80 && current_packet[12] == 0x47 && pos >= TS_PACKET_SIZE) {
+        avio_seek(pb, 12 - TS_PACKET_SIZE, SEEK_CUR);
         return 0;
     }