Tidy up sys_setgroups and kern_setgroups.

- 'groups' initialization to NULL is always ovewrwriten before use, so plug it
- get rid of 'goto out'
- kern_setgroups's callers already validate ngrp, so only assert the condition
- ngrp  is an u_int, so 'ngrp < 1' is more readable as 'ngrp == 0'

No functional changes.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273685 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f