Fix up an assertion in kern_setgroups, it should compare with ngroups_max + 1

Bug introdued in r273685.

Noted by: Tiwei Bie <btw mail.ustc.edu.cn>


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273691 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f