s3-winbind: don't try to do clever thing if the username is not found while authentic...

s3-winbind: don't try to do clever thing if the username is not found while authenticating through winbind

This could cause that we authenticate a user with a bogus domain to
winbind's domain if the password supplied for the PAM_AUTH match.

The problem was reported by Jeff Venable (jvenable@juniper.net).
Patch from Andrew Bartlett (abartlett@samba.org).

Autobuild-User: Matthieu Patou <mat@samba.org>
Autobuild-Date: Mon Jan 30 18:58:12 CET 2012 on sn-devel-104
(cherry picked from commit 56d5cb938651b9c67a8400d1adc61a23889a6a29)

Fix bug #8734 (When using PAM_AUTH API from winbind if Kerberos auth is enabled,
samba will authenticate user with a bogus domain).