fix integer underflow in getgrnam_r and getpwnam_r

fix integer underflow in getgrnam_r and getpwnam_r

Sometimes nscd(8) will return a 1-byte buffer for a nonexistent entry. This
triggered an integer underflow in grp_unmarshal_func, causing getgrnam_r to
return ERANGE instead of 0.

Fix the user's buffer size check, and add a correct check for a too-small
nscd buffer.

PR:		248932
Event:		September 2020 Bugathon
Reviewed by:	markj
MFC after:	2 weeks
Sponsored by:	Axcient
Differential Revision: https://reviews.freebsd.org/D26204