Restore correct behavior of 0.3.0.4-rc with bridges+ipv6-min

In that chutney test, the bridge client is configured to connect to
the same bridge at 127.0.0.1:5003 _and_ at [::1]:5003, with no
change in transports.

That meant, I think, that the descriptor is only assigned to the
first bridge when it arrives, and never the second.