Commit b85207c8e9 for ffmpeg

commit b85207c8e9c1c45b810530563820df0726f88c0d
Author: Michael Niedermayer <michael@niedermayer.cc>
Date:   Mon Oct 19 02:14:25 2026 +0200

    avformat/rtpenc: Reject packet sizes too small for the MPEG video payload header

    Fixes: negative-size-param
    Fixes: infinite loop
    Fixes: vrmb3QRC6jK1
    Fixes: swarm-Signed-integer-underflow-in-RTP-MPEG-video-packet-length-4acadd
    Fixes: ffmpeg -f lavfi -i color=size=64x64:duration=1 -packetsize 13 -c:v mpeg1video -f rtp rtp://127.0.0.1:45004

    Negative-size memcpy and endless loop Replicated through UnModified FFmpeg with ASAN

    Found-by: zhang xingxing <2388969553@qq.com>

diff --git a/libavformat/rtpenc.c b/libavformat/rtpenc.c
index 24bf6b93f8..471c988d9c 100644
--- a/libavformat/rtpenc.c
+++ b/libavformat/rtpenc.c
@@ -178,6 +178,7 @@ static int rtp_write_header(AVFormatContext *s1)
         break;
     case AV_CODEC_ID_MPEG1VIDEO:
     case AV_CODEC_ID_MPEG2VIDEO:
+        payload_header_size = 4;
         break;
     case AV_CODEC_ID_MPEG2TS:
         if (s->max_payload_size < TS_PACKET_SIZE) {