Loss of MySQL CB connection causes Kea to segfault

This is very easy to reproduce:

1. Configure kea-dhcp4 with valid MySQL CB parameters
2. Start kea, with MySQL up and running
3. After successful startup, stop MySQL
4. The first attempt to refresh CB detects a loss of connectivity
5. The second attempt segfaults. It appears to be trying to use a no longer viable connection

I did this with kea-dhcp4 but I am confident kea-dhcp6 crashes the same way. Apparently after the failure in step 4 we are not flagging the connection as failed?

The stack trace is attached

mysql_cb_backtrace.txt