fix: change to pointer of interface

Author: derrandz

conn.go

@@ -349,8 +349,8 @@ func (c *Conn) WritePacket(b []byte) (icmp []byte, err error) {
 		return nil, nil
 	}
 	if err := c.str.SendDatagram(data); err != nil {
-		errDTL := &quic.DatagramTooLargeError{}
-		if errors.As(err, errDTL) {
+		var errDTL *quic.DatagramTooLargeError
+		if errors.As(err, &errDTL) {
 			icmpPacket, err := composeICMPTooLargePacket(b, minMTU)
 			if err != nil {
 				log.Printf("failed to compose ICMP too large packet: %s", err)