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 620a6a54bb7bb6e1c5607092b6ec49e353e0925f)