Protect against segfault in remove_ldap_connection_callbacks
    
    If sdap_mark_offline() is called before a live connection is
    established, sdap_fd_events could be NULL, causing a segfault when
    remove_ldap_connection_callbacks() attempts to free the
    sdap_fd_events->conncb
    
    https://fedorahosted.org/sssd/ticket/545