[lightning-net-tokio] Fix race-y unwrap fetching peer socket address

[lightning-net-tokio] Fix race-y unwrap fetching peer socket address

I recently saw the following panic on one of my test nodes:

```
thread 'tokio-runtime-worker' panicked at 'called `Result::unwrap()`
    on an `Err` value: Os { code: 107, kind: NotConnected, message:
    "Transport endpoint is not connected" }',
    rust-lightning/lightning-net-tokio/src/lib.rs:250:38
```

Presumably what happened is somehow the connection was closed in
between us accepting it and us going to start processing it. While
this is a somewhat surprising race, its clearly reachable. The fix
proposed here is quite trivial - simply don't `unwrap` trying to
fetch our peer's socket address, instead treat the peer address as
`None` and discover the disconnection later when we go to read.