memberof: check for empty arrays to avoid segfaults
    
    The arrays with members to add or delete may be empty, i.e. have 0
    entries. In this case further processing should be skipped to avoid
    segfaults later on.
    
    Fixes (hopefully) https://fedorahosted.org/sssd/ticket/2430
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>