passdb: fix NT_STATUS_NO_SUCH_GROUP

passdb: fix NT_STATUS_NO_SUCH_GROUP

Share options like "force group" and "valid users = @group1"
triggered a NT_STATUS_NO_SUCH_GROUP. While the group was found in
the SAM backend, its objectclass was not retrived.

This fix also revealed a talloc access after free in the group
branch of pdb_samba_dsdb_getgrfilter.

[Bug 9570] Access failure for shares with "force group" or "valid users = @group"

https://bugzilla.samba.org/show_bug.cgi?id=9570

Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>