openssh: Fix pre-authentication remote code execution in sshd.

Reported by: Qualys Threat Research Unit (TRU)
Approved by: so
Security: FreeBSD-SA-24:04.openssh
Security: CVE-2024-6387

(cherry picked from commit 2abea9df01655633aabbb9bf3204c90722001202)
(cherry picked from commit 25cf430cd551158135ef6f0c1aadf273ff3430c9)