Fixed KeySharesParams reset on subsequent ApplyPreset calls

* This fixes an issue where servers that selected a key_share other than
  X25519 would cause TLS handshake to fail. This is because
  the precomputed key_shares are thrown away and not replaced if
  ApplyPreset is called more than once.

u_parrots.go

@@ -2636,7 +2636,12 @@ func (uconn *UConn) ApplyPreset(p *ClientHelloSpec) error {
 	} else if kemKey, ok := clientKeySharePrivate.(*kemPrivateKey); ok {
 		uconn.HandshakeState.State13.KEMKey = kemKey.ToPublic()
 	}
-	uconn.HandshakeState.State13.KeySharesParams = NewKeySharesParameters()
+
+	// [Psiphon]
+	if uconn.HandshakeState.State13.KeySharesParams == nil {
+		uconn.HandshakeState.State13.KeySharesParams = NewKeySharesParameters()
+	}
+
 	hello := uconn.HandshakeState.Hello
 
 	switch len(hello.Random) {