SECURITY UPDATE: use-after-free in channel_rcv_data() when a DATA packet
is received on a channel already closed by the remote peer
debian/patches/CVE-2026-59850.patch: reject DATA packets on channels
already closed by the remote peer (SSH_CHANNEL_FLAG_CLOSED_REMOTE) in
channel_rcv_data(), and additionally drop the callback list in
ssh_channel_free() so DATA arriving between a local free and the
peer's CLOSE can no longer dispatch callbacks on a released channel,
both in src/channels.c