lavf/flvdec: Allow files where the PreviousTagSize is not set according to the spec.

Some muxer use the FLV field PreviousTagSize to be the sum of tag
length. Without this change, the flv demuxer think the file is broken
and the re-sync will fail.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/flvdec.c

@@ -60,6 +60,7 @@ typedef struct FLVContext {
     uint8_t resync_buffer[2*RESYNC_BUFFER_SIZE];
 
     int broken_sizes;
+    int sum_flv_tag_size;
 } FLVContext;
 
 static int probe(AVProbeData *p, int live)
@@ -629,6 +630,7 @@ static int flv_read_metabody(AVFormatContext *s, int64_t next_pos)
 static int flv_read_header(AVFormatContext *s)
 {
     int offset, flags;
+    FLVContext *flv = s->priv_data;
 
     avio_skip(s->pb, 4);
     flags = avio_r8(s->pb);
@@ -649,6 +651,7 @@ static int flv_read_header(AVFormatContext *s)
     avio_skip(s->pb, 4);
 
     s->start_time = 0;
+    flv->sum_flv_tag_size = 0;
 
     return 0;
 }
@@ -857,6 +860,7 @@ retry:
         type = (avio_r8(s->pb) & 0x1F);
         orig_size =
         size = avio_rb24(s->pb);
+        flv->sum_flv_tag_size += size + 11;
         dts  = avio_rb24(s->pb);
         dts |= (unsigned)avio_r8(s->pb) << 24;
         av_log(s, AV_LOG_TRACE, "type:%d, size:%d, last:%d, dts:%"PRId64" pos:%"PRId64"\n", type, size, last, dts, avio_tell(s->pb));
@@ -1141,7 +1145,7 @@ retry_duration:
 leave:
     last = avio_rb32(s->pb);
     if (last != orig_size + 11 &&
-        (last != orig_size || !last) &&
+        (last != orig_size || !last) && last != flv->sum_flv_tag_size &&
         !flv->broken_sizes) {
         av_log(s, AV_LOG_ERROR, "Packet mismatch %d %d\n", last, orig_size + 11);
         avio_seek(s->pb, pos + 1, SEEK_SET);