Ticket 49291 - slapi_search_internal_callback_pb may SIGSEV if related pblock has not operation set
    
    Bug Description:
        if slapi_search_internal_set_pb is called with an invalid (NULL) base, the pblock should not
        be used to call send_ldap_result. If it is, the send_ldap_result trying to derefence the
        operation pointer will crash
    
    Fix Description:
        Check that the operation is set before derefencing it
    
    https://pagure.io/389-ds-base/issue/49291
    
    Reviewed by: Mark Reynolds
    
    Platforms tested: F23
    
    Flag Day: no
    
    Doc impact: no