Ignore "use of closed network connection" errors

Every time the server returns a response to the client it calls
BuildAndSend(), which assumes that the other side is still listening
and returns a "use of closed network connection" error when it does
not. This error always shows up when the client disconnects first,
which is totally normal, but messes up logs and fires stale
alarms. This commit silences these errors.

I am fairly certain that this change does not break anything. Yet,
please give feedback.

internal/server/util.go

@@ -2,6 +2,7 @@ package server
 
 import (
 	"crypto/md5" //nolint:gosec,gci
+	"errors"
 	"fmt"
 	"io"
 	"math/rand"
@@ -16,7 +17,6 @@ import (
 const (
 	maximumAllocationLifetime = time.Hour // https://tools.ietf.org/html/rfc5766#section-6.2 defines 3600 seconds recommendation
 	nonceLifetime             = time.Hour // https://tools.ietf.org/html/rfc5766#section-4
-
 )
 
 func randSeq(n int) string {
@@ -46,6 +46,10 @@ func buildAndSend(conn net.PacketConn, dst net.Addr, attrs ...stun.Setter) error
 		return err
 	}
 	_, err = conn.WriteTo(msg.Raw, dst)
+	if errors.Is(err, net.ErrClosed) {
+		return nil
+	}
+
 	return err
 }