Transport: Add REALITY TLS1.3 support warning (XTLS#3774)

Author: Fangliding

transport/internet/reality/reality.go

@@ -140,6 +140,9 @@ func UClient(c net.Conn, config *Config, ctx context.Context, dest net.Destinati
 		if err != nil {
 			return nil, errors.New("REALITY: publicKey == nil")
 		}
+		if uConn.HandshakeState.State13.EcdheKey == nil {
+			return nil, errors.New("Current fingerprint ", uConn.ClientHelloID.Client, uConn.ClientHelloID.Version, " does not support TLS 1.3, REALITY handshake cannot establish.")
+		}
 		uConn.AuthKey, _ = uConn.HandshakeState.State13.EcdheKey.ECDH(publicKey)
 		if uConn.AuthKey == nil {
 			return nil, errors.New("REALITY: SharedKey == nil")