winbindd: handling missing idmap in getgrgid()

winbindd: handling missing idmap in getgrgid()

A similar hunk was added via commit
89f753c1fc824fef29aebb7d783ab7e09cd1f04e ("winbind: Use xids2sids in getpwuid"),
but it was missing in commit
e2dda192e7f8b65a5f02120be56cf0f07d03679f ("winbind: Use xids2sids in getgrgid")

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14265

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Wed Feb  5 17:56:58 UTC 2020 on sn-devel-184

(cherry picked from commit 4d0bda9467ac3f45f85f48a281cdb173ce1064eb)

Autobuild-User(v4-10-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-10-test): Tue Feb 11 12:26:52 UTC 2020 on sn-devel-144