Ticket 580 - Wrong error code return when using EXTERNAL SASL and no client certificate
    
    Bug Description:  When doing a SASL EXTERNAL bind, and no client certificate is provided,
                      the bind succeeds(as anonymous).  However, the bind should fail with
                      an error 48.
    
    Fix Description:  Check for the missing client certificate and return the appropriate
                      error.
    
    https://fedorahosted.org/389/ticket/580
    
    Reviewed by: Noriko(Thanks!)