IFP: Skip non-POSIX groups properly
    
    When ifp_users_user_get_groups is called, for example via GetAll and
    the list of groups contains a non-POSIX group, we skip an array member,
    resulting in random memory being passed to the caller.
    
    Resolves:
        https://fedorahosted.org/sssd/ticket/2863
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>