Commit 2c515eea07 for ffmpeg

commit 2c515eea07df719379e7f73deeeb7ccb8768433f
Author: Michael Niedermayer <michael@niedermayer.cc>
Date:   Sat Oct 10 07:25:28 2026 +0200

    avcodec/cbs_sei: clear the message slot handed out by ff_cbs_sei_list_add()

    Fixes: heap-use-after-free
    Fixes: YtyT0QH1InrQ
    Fixes: ANT-2026-13DK7NZD
    Fixes: poc.264
    Found-by: Claude and Ada Logics

    Use after free Replicated through UnModified FFmpeg with ASAN

diff --git a/libavcodec/cbs_sei.c b/libavcodec/cbs_sei.c
index f354a10029..4f04b9ff83 100644
--- a/libavcodec/cbs_sei.c
+++ b/libavcodec/cbs_sei.c
@@ -279,11 +279,8 @@ int ff_cbs_sei_list_add(SEIRawMessageList *list)

         list->messages = ptr;
         list->nb_messages_allocated = new_count;
-
-        // Zero the newly-added entries.
-        memset(list->messages + old_count, 0,
-               (new_count - old_count) * sizeof(*list->messages));
     }
+    memset(&list->messages[list->nb_messages], 0, sizeof(*list->messages));
     ++list->nb_messages;
     return 0;
 }