nfsuserd: Fix OOB access on membership of too many groups, take 2

Forgot to reset 'ngroup' after one of the calls to getgrouplist() in the
previous commit with same title (referenced also below).

MFC after: 1 hour
Fixes: e6c623c86ab4 ("Add support for the "-manage-gids" option to the nfsuserd daemon.")
Fixes: bb339adfb2a2 ("nfsuserd: Fix OOB access on membership of too many groups")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 75eb6846269bb9bec1eaf4f0ad93d7525715b550)