Fix data race in WriteTo

There is no harm in shadowing `err` in the goroutine since we do not
need it outside of the routine.

I also discovered data races in tcp_conn_test.go while fixing this bug
and have included them in this PR.

internal/client/tcp_conn_test.go

@@ -92,12 +92,12 @@ func TestTCPConn(t *testing.T) {
 		client = &mockClient{
 			performTransaction: func(msg *stun.Message, _ net.Addr, _ bool) (TransactionResult, error) {
 				if msg.Type.Class == stun.ClassRequest && msg.Type.Method == stun.MethodConnect {
-					msg, err = stun.Build(
+					msg, buildErr := stun.Build(
 						stun.TransactionID,
 						stun.NewType(stun.MethodConnect, stun.ClassErrorResponse),
 						stun.ErrorCodeAttribute{Code: stun.CodeBadRequest},
 					)
-					assert.NoError(t, err)
+					assert.NoError(t, buildErr)
 
 					return TransactionResult{Msg: msg}, nil
 				}
@@ -174,12 +174,12 @@ func TestTCPConn(t *testing.T) {
 					typ = stun.NewType(stun.MethodCreatePermission, stun.ClassSuccessResponse)
 				}
 
-				msg, err = stun.Build(
+				msg, buildErr := stun.Build(
 					stun.TransactionID,
 					typ,
 					cid,
 				)
-				assert.NoError(t, err)
+				assert.NoError(t, buildErr)
 
 				return TransactionResult{Msg: msg}, nil
 			},

internal/client/udp_conn.go

@@ -240,9 +240,8 @@ func (c *UDPConn) WriteTo(payload []byte, addr net.Addr) (int, error) { //nolint
 		if bound.state() == bindingStateReady && time.Since(bound.refreshedAt()) > 5*time.Minute {
 			bound.setState(bindingStateRefresh)
 			go func() {
-				err = c.bind(bound)
-				if err != nil {
-					c.log.Warnf("Failed to bind() for refresh: %s", err)
+				if bindErr := c.bind(bound); bindErr != nil {
+					c.log.Warnf("Failed to bind() for refresh: %s", bindErr)
 					bound.setState(bindingStateFailed)
 					// Keep going...
 				} else {

lt_cred.go

@@ -3,7 +3,7 @@
 
 package turn
 
-import ( //nolint:gci
+import (
 	"crypto/hmac"
 	"crypto/sha1" //nolint:gosec,gci
 	"encoding/base64"