openssh: Handle localtime_r() failure by return "UNKNOWN-TIME"

Apply openssh-portable commit 8b6c1f402feb by deraadt@openbsd.org

Approved by: re (cperciva)
Obtained from: openssh-portable
PR: 288773
Reported by: wosch
Sponsored by: The FreeBSD Foundation

(cherry picked from commit ce03706ab26c5770150f1ef96aca36b69baa535f)
(cherry picked from commit 0a45aa9e954acc75484d59ad42ee440aa7f034c7)