fix error assertion for too large datagrams (#45)

* use errors.As

* Update .gitignore

Co-authored-by: Marten Seemann <[email protected]>

* make linter happy

* fix: change to pointer of interface

---------

Co-authored-by: derrandz <[email protected]>

Author: marten-seemann

conn.go

@@ -349,7 +349,8 @@ func (c *Conn) WritePacket(b []byte) (icmp []byte, err error) {
 		return nil, nil
 	}
 	if err := c.str.SendDatagram(data); err != nil {
-		if errors.Is(err, &quic.DatagramTooLargeError{}) {
+		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)