Certification information不能过大

/*
If certification information is too big this event can't be transmitted
as it would cause failures on all group members.
To avoid this, we now instead encode an error that will make the joiner
leave the group.
*/
if (event_size > get_slave_max_allowed_packet()) {
cert_info.clear();
cert_info[Certifier::CERTIFICATION_INFO_ERROR_NAME] =
"Certification information is too large for transmission.";
vchange_event->set_certification_info(&cert_info, &event_size);
}