Revert "protect socket variable with mutex; fix #437"

This reverts commit 1b7ce4c378.

frontend/udp/frontend.go

@@ -49,7 +49,6 @@ func (cfg Config) LogFields() log.Fields {
 // Frontend holds the state of a UDP BitTorrent Frontend.
 type Frontend struct {
 	socket  *net.UDPConn
-	m       sync.Mutex
 	closing chan struct{}
 	wg      sync.WaitGroup
 
@@ -104,20 +103,10 @@ func (t *Frontend) Stop() stop.Result {
 
 	c := make(stop.Channel)
 	go func() {
-		t.m.Lock()
-		defer t.m.Unlock()
-
 		close(t.closing)
-		if t.socket != nil {
-			t.socket.SetReadDeadline(time.Now())
-		}
+		t.socket.SetReadDeadline(time.Now())
 		t.wg.Wait()
-
-		var err error
-		if t.socket != nil {
-			err = t.socket.Close()
-		}
-		c.Done(err)
+		c.Done(t.socket.Close())
 	}()
 
 	return c.Result()
@@ -131,9 +120,7 @@ func (t *Frontend) listenAndServe() error {
 		return err
 	}
 
-	t.m.Lock()
 	t.socket, err = net.ListenUDP("udp", udpAddr)
-	t.m.Unlock()
 	if err != nil {
 		return err
 	}