use pointer to byte slices in the buffer pool

https://staticcheck.io/docs/staticcheck#SA6002 suggests to use pointers
to objects in the sync.Pool.

packet_packer.go

@@ -342,8 +342,8 @@ func (p *packetPacker) writeAndSealPacket(
 	payloadFrames []wire.Frame,
 	sealer handshake.Sealer,
 ) ([]byte, error) {
-	raw := getPacketBuffer()
-	buffer := bytes.NewBuffer(raw)
+	raw := *getPacketBuffer()
+	buffer := bytes.NewBuffer(raw[:0])
 
 	if err := header.Write(buffer, p.perspective, p.version); err != nil {
 		return nil, err