stop using the ExtendedHeader for parsing short header packets in tests

fuzzing/header/fuzz.go

@@ -42,6 +42,11 @@ func Fuzz(data []byte) int {
 		panic("inconsistent 0-RTT packet detection")
 	}
 
+	if !wire.IsLongHeaderPacket(data[0]) {
+		wire.ParseShortHeader(data, connIDLen)
+		return 1
+	}
+
 	var extHdr *wire.ExtendedHeader
 	// Parse the extended header, if this is not a Retry packet.
 	if hdr.Type == protocol.PacketTypeRetry {
@@ -58,9 +63,6 @@ func Fuzz(data []byte) int {
 	if hdr.IsLongHeader && hdr.Length > 16383 {
 		return 1
 	}
-	if !hdr.IsLongHeader {
-		return 1
-	}
 	b := &bytes.Buffer{}
 	if err := extHdr.Write(b, version); err != nil {
 		// We are able to parse packets with connection IDs longer than 20 bytes,