Ticket 571 - server does not accept 0 length LDAP Control sequence
    
    Bug Description: If we receive an incorrect control, with non NULL bv_val but
                     bv_len=0, it can trigger a crash.  This bug occurs because
                     of a call of openldap ber_init with such incorrect control.
    
    Fix Description: Before calling ber_init, check that the control length is not 0.
    
    https://fedorahosted.org/389/ticket/571
    
    Reviewed by richm(Thanks!)