ssh: fix OpenSSH 9.4 regression with multiplexed sessions

Upstream commit message:
upstream: fix regression in OpenSSH 9.4 (mux.c r1.99) that caused
multiplexed sessions to ignore SIGINT under some circumstances.
Reported by / feedback naddy@, ok dtucker@

OpenBSD-Commit-ID: 4d5c6c894664f50149153fd4764f21f43e7d7e5a

Fixes: 535af610a4fd ("ssh: Update to OpenSSH 9.4p1")
Obtained from: OpenSSH 803e22eabd3b
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 1b91d634a58c45d7b33dee044034260f747c80d9)