s3-winbind: Don't fail on users without a uid.

s3-winbind: Don't fail on users without a uid.

This fixes bug #8608.

If you join samba with idmap_ad backend to an AD. When you try to
enumerate users with 'getent passwd' and the user doesn't have a uid
set, then getent is aborted cause of NT_STATUS_NONE_MAPPED. If we can't
map a user we should not stop but continue enumerating users.

This normally happens with the default user 'krbtgt' with idmap_ad but
could also happen with other backends.

Autobuild-User: Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date: Tue Nov 15 16:52:04 CET 2011 on sn-devel-104

(backported from commit 10b285ccc29b106f164a6c18116e237634867717)

Signed-off-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit a64f88f4e97f0f9c006fe6f0cefc0a61efc1ca98)