nsswitch: Skip groups we were not able to map.

nsswitch: Skip groups we were not able to map.

If we have configured the idmap_ad backend it is possible that the user
is in a group without a gid set. This will result in (uid_t)-1 as the
gid. We return this invalid gid to NSS which is wrong.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=10824

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: David Disseldorp <ddiss@samba.org>
Autobuild-User(master): David Disseldorp <ddiss@samba.org>
Autobuild-Date(master): Fri Sep 19 17:57:14 CEST 2014 on sn-devel-104

(cherry picked from commit 7f59711f076e98ece099f6b38ff6da8c80fa6d5e)
Signed-off-by: Andreas Schneider <asn@samba.org>
Autobuild-User(v4-0-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-0-test): Mon Sep 29 22:07:06 CEST 2014 on sn-devel-104