getgrent.c: adjust _nextypgroup() slightly so that it continues processing
the group map after encountering a badly formatted entry.

getpwent.c: same as above for _nextyppass(), and also turn a couple of
sprintf()s into snprintf()s to avoid potential buffer overruns. (The
other day I nearly went mad because of a username in my NIS database
that's actually 9 characters long instead of 8. Stuffing a 9-character
username into an 8-character buffer can do some strange things.)

(This reminds me: I hope somebody's planning to fix the buffer overrun
security hole in syslog(3) before 2.1 ships.)