mainloop: don't panic when discovery stops without credentials

[Hayao0819]

spotifyd panics with a bare Option::unwrap() when the zeroconf responder fails to come up. My service sat in a systemd restart loop with nothing but this in the journal:

spotifyd[1029740]: The application panicked (crashed).
spotifyd[1029740]: Message:  called `Option::unwrap()` on a `None` value
spotifyd[1029740]: Location: src/main_loop.rs:244
systemd[1]: spotifyd.service: Main process exited, code=exited, status=101/n/a
systemd[1]: spotifyd.service: Scheduled restart job, restart counter is at 78.

The cause was net.ipv4.igmp_max_memberships, default 20, against the 27 interfaces carrying addresses on this box (docker, tailscale, a pile of emulator taps). libmdns gives up joining the multicast group, and spotifyd dies two lines later:

[INFO] Starting zeroconf server to advertise on local network.
[ERROR] libmdns error: Setting up dns-sd failed: No buffer space available (os error 105)
The application panicked (crashed).
Message:  called `Option::unwrap()` on a `None` value
Location: src/main_loop.rs:52

(:52 is current master, :244 is the same code in 0.4.1.)

librespot does report the failure, we just never see it. launch_libmdns logs it and pushes it into the event channel (discovery/src/lib.rs#L418-L421):

        if let Err(e) = inner() {
            log::error!("libmdns error: {e}");
            let _ = status_tx.send(DiscoveryEvent::ZeroconfError(e));
        }

The Stream impl then turns that into end-of-stream, since Item is Credentials and there's nowhere to put an error (discovery/src/lib.rs#L539-L553):

impl Stream for Discovery {
    type Item = Credentials;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        match Pin::new(&mut self.event_rx).poll_recv(cx) {
            // Yields credentials
            Poll::Ready(Some(DiscoveryEvent::Credentials(creds))) => Poll::Ready(Some(creds)),
            // Also terminate the stream on fatal server or MDNS/DNS-SD errors.
            Poll::Ready(Some(
                DiscoveryEvent::ServerError(_) | DiscoveryEvent::ZeroconfError(_),
            )) => Poll::Ready(None),

All of it happens inside spawn_blocking, long after launch() handed us an Ok, so the retry loop in setup.rs never gets a chance either.

None here means "no credentials, ever", not "shutting down". Handling it gives:

Error:
   0: failed to connect to spotify
   1: Discovery stopped without providing credentials.

Suggestion: Check the log for zeroconf errors, or log in with `spotifyd authenticate` so that spotifyd doesn't depend on discovery.

Only a discovery-only start with no cached credentials reaches that branch. With credentials present, get_credentials takes the non-blocking arm and behaves as before.