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.

mint_utils.go

@@ -139,8 +139,8 @@ func unpackInitialPacket(aead crypto.AEAD, hdr *wire.Header, data []byte, versio
 // packUnencryptedPacket provides a low-overhead way to pack a packet.
 // It is supposed to be used in the early stages of the handshake, before a session (which owns a packetPacker) is available.
 func packUnencryptedPacket(aead crypto.AEAD, hdr *wire.Header, f wire.Frame, pers protocol.Perspective) ([]byte, error) {
-	raw := getPacketBuffer()
-	buffer := bytes.NewBuffer(raw)
+	raw := *getPacketBuffer()
+	buffer := bytes.NewBuffer(raw[:0])
 	if err := hdr.Write(buffer, pers, hdr.Version); err != nil {
 		return nil, err
 	}