avformat/mpegts: correctly skip TP_extra_header in m2ts

instead of just resyncing and skipping a bunch of TS packets, leading to
a loss of frames.

Before this, a stray byte with the value of 0x47 in TP_extra_header
would throw off the detection of where TS packets start.

A typical file that could cause issues would look like this:

    00000300: 238f 4780 4750 1110 0000 01e0 0000 84c0
                   ^^   ^^
The first four bytes here are TP_extra_header and the actual TS packet
starts at offset 0x304

FFmpeg would try to read a packet at 0x300 but since nothing skips the
4 byte TP_extra_header, find that the first byte is not 0x47 and
immediately go into mpegts_resync, and incorrectly detect the stray 0x47
in the TP_extra_header at 0x302 as the new sync byte.

Fix this by correctly skipping the first 4 bytes if the source packet
is 192 bytes.

Signed-off-by: llyyr <llyyr.public@gmail.com>
Signed-off-by: Hendrik Leppkes <h.leppkes@gmail.com>

libavformat/mpegts.c

@@ -2961,6 +2961,12 @@ static int read_packet(AVFormatContext *s, uint8_t *buf, int raw_packet_size,
     AVIOContext *pb = s->pb;
     int len;
 
+    // 192 bytes source packet that start with a 4 bytes TP_extra_header
+    // followed by 188 bytes of TS packet. The sync byte is at offset 4, so skip
+    // the first 4 bytes otherwise we'll end up syncing to the wrong packet.
+    if (raw_packet_size == TS_DVHS_PACKET_SIZE)
+        avio_skip(pb, 4);
+
     for (;;) {
         len = ffio_read_indirect(pb, buf, TS_PACKET_SIZE, data);
         if (len != TS_PACKET_SIZE)
@@ -2983,7 +2989,11 @@ static int read_packet(AVFormatContext *s, uint8_t *buf, int raw_packet_size,
 static void finished_reading_packet(AVFormatContext *s, int raw_packet_size)
 {
     AVIOContext *pb = s->pb;
-    int skip = raw_packet_size - TS_PACKET_SIZE;
+    int skip;
+    if (raw_packet_size == TS_DVHS_PACKET_SIZE)
+        skip = raw_packet_size - TS_DVHS_PACKET_SIZE;
+    else
+        skip = raw_packet_size - TS_PACKET_SIZE;
     if (skip > 0)
         avio_skip(pb, skip);
 }