userdb: reset errno before getpwent()

userdb: reset errno before getpwent()

errno handling for NSS is always a bit weird since NSS modules generally
are not particularly careful with it. Hence let's initialize errno
explicitly before we invoke getpwent() so that we know it's in a
reasonable state afterwards on failure, or zero if not.

We do this in most places we use NSS, including in userdb when it comes
to getgrent(), just for getpwent() we don't so far. Address that.