avformat/rtpenc_vc2hq: Write the extended sequence number

RFC 8450 requires the high order 16 bits of the 32 bit packet sequence
number in the payload header, 0 was always written. Count the wraps of
the RTP sequence number in the RTP muxer.

Not a security issue.

Found during triage of the security report HRhmcrDPRPyf

libavformat/rtpenc.c

@@ -384,6 +384,8 @@ void ff_rtp_send_data(AVFormatContext *s1, const uint8_t *buf1, int len, int m)
     avio_flush(s1->pb);
 
     s->seq = (s->seq + 1) & 0xffff;
+    if (!s->seq)
+        s->seq_high++;
     s->octet_count += len;
     s->packet_count++;
 }

libavformat/rtpenc.h

@@ -32,6 +32,7 @@ struct RTPMuxContext {
     uint32_t ssrc;
     const char *cname;
     int seq;
+    uint16_t seq_high;
     uint32_t timestamp;
     uint32_t base_timestamp;
     uint32_t cur_timestamp;

libavformat/rtpenc_vc2hq.c

@@ -43,7 +43,7 @@ static int send_packet(AVFormatContext *ctx, uint8_t parse_code, int info_hdr_si
         return AVERROR_INVALIDDATA;
     }
 
-    AV_WB16(&rtp_ctx->buf[0], 0); /* extended sequence number */
+    AV_WB16(&rtp_ctx->buf[0], rtp_ctx->seq_high); /* extended sequence number */
     AV_WB8 (&rtp_ctx->buf[2], i ? (f ? (0x03) : (0x02)) : 0x00); /* flags: interlaced, second field */
     AV_WB8 (&rtp_ctx->buf[3], parse_code);
     if (size > 0)