fix(relay): prevent panic on double Stop() of UDPProxyServer (#51)

[Adityakk9031]

Summary

Fixes #51

Gerbil was panicking with panic: close of closed channel on every clean
shutdown (and sometimes randomly during operation). The process would then
restart via Docker's restart policy, re-register with Pangolin, and resume —
but left some resources in a broken state requiring manual intervention.

---

Root Cause

UDPProxyServer.Stop() was being called twice on every shutdown:

Call site	File	Trigger
defer proxyRelay.Stop()	main.go:533	Always, when main() returns
proxyRelay.Stop()	main.go:606	When groupCtx is cancelled (shutdown signal)

Both fire on every clean exit. The second call panicked because
close(s.packetChan) was executed twice.

The existing guard was ineffective:

// OLD — broken guard
select {
case <-s.ctx.Done():  // ctx is ALREADY done on the 2nd call too
default:
}
close(s.packetChan)   // executes BOTH times → panic

[Adityakk9031]

@oschwartz10612 and @miloschwartz have a look