SECURITY UPDATE: Use-after-free of a pending dedicated ACK buffer when a
TLS session is freed during session promotion or expiry
debian/patches/CVE-2026-12996.patch: add the check_session_buf_not_used()
safeguard, including its ks->ack_write_buf check, and call it before
every site in tls_multi_process() that frees or resets a session
CVE-2026-12996
the safeguard does not exist in 2.4.7, so it is introduced here already
in its post-CVE-2026-12996 form; as a result this patch also fixes
CVE-2026-40215, which shares the same safeguard