Perform SnowflakeConn.Close() logic only once

Use synchronization to avoid a panic if SnowflakeConn.Close is called
more than once.

client/lib/peers.go

@@ -29,6 +29,7 @@ type Peers struct {
 	melt chan struct{}
 
 	collectLock sync.Mutex
+	closeOnce   sync.Once
 }
 
 // NewPeers constructs a fresh container of remote peers.
@@ -122,6 +123,7 @@ func (p *Peers) purgeClosedPeers() {
 // End closes all active connections to Peers contained here, and stops the
 // collection of future Peers.
 func (p *Peers) End() {
+	p.closeOnce.Do(func() {
 		close(p.melt)
 		p.collectLock.Lock()
 		defer p.collectLock.Unlock()
@@ -135,4 +137,5 @@ func (p *Peers) End() {
 			e = next
 		}
 		log.Printf("WebRTC: melted all %d snowflakes.", cnt)
+	})
 }