Fix panic when error occurs binding uTP socket

Fixes anacrolix#187.

Author: anacrolix

client_test.go

@@ -1102,3 +1102,15 @@ func TestMultipleTorrentsWithEncryption(t *testing.T) {
 	tr.DownloadAll()
 	client.WaitAll()
 }
+
+func TestClientAddressInUse(t *testing.T) {
+	s, _ := NewUtpSocket("udp", ":50007")
+	if s != nil {
+		defer s.Close()
+	}
+	cfg := TestingConfig()
+	cfg.ListenAddr = ":50007"
+	cl, err := NewClient(cfg)
+	require.Error(t, err)
+	require.Nil(t, cl)
+}

utp_go.go

@@ -7,5 +7,10 @@ import (
 )
 
 func NewUtpSocket(network, addr string) (utpSocket, error) {
-	return utp.NewSocket(network, addr)
+	s, err := utp.NewSocket(network, addr)
+	if s == nil {
+		return nil, err
+	} else {
+		return s, err
+	}
 }

utp_libutp.go

@@ -7,5 +7,10 @@ import (
 )
 
 func NewUtpSocket(network, addr string) (utpSocket, error) {
-	return utp.NewSocket(network, addr)
+	s, err := utp.NewSocket(network, addr)
+	if s == nil {
+		return nil, err
+	} else {
+		return s, err
+	}
 }

utp_test.go

@@ -0,0 +1,15 @@
+package torrent
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewUtpSocketErrorNilInterface(t *testing.T) {
+	s, err := NewUtpSocket("fix", "your:language")
+	assert.Error(t, err)
+	if s != nil {
+		t.Fatalf("expected nil, got %#v", s)
+	}
+}