Only test the return value in mktime_negyear

Testing for the errno is an optional requirement according to POSIX, and
FreeBSD doesn't document that errno would be set on failure with mktime

Submitted by: pho
Sponsored by: EMC / Isilon Storage Division


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273024 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f