v0.4 fails with “Address not available”

Since v0.4 chatmail-turn fails to start with this error message:

```
Listening on 172.31.1.1
Error: Io(IoError(Os { code: 99, kind: AddrNotAvailable, message: "Address not available" }))
```

There is no such address configured on this server, however `ip` shows the following:

```
# ip a
[…]
2: ens3: mtu 1500 qdisc fq_codel state UP group default qlen 1000
[…]
inet […] peer 172.31.1.1/32 brd […] scope global ens3
valid_lft forever preferred_lft forever
[…]
# ip route
default via 172.31.1.1 dev ens3 onlink
172.31.1.1 dev ens3 proto kernel scope link src […]
```

This is a virtual server at Hetzner.

v0.3 works fine.