A single remaining master is valid for readconnroute configured with 'router_options=slave'

With a configuration like

[SomeService]

type=service

router=readconnroute

router_options=slave

readconnroute will still select a master if that is the only server that is left.

However, later, from 2.1.13 onwards, when readconnroute asseses in connection_is_valid() whether the connection is valid, it will reject it and thus the client will get no connection.