Fixed remote login

I forgot that this message sometimes receives a string instead of a buffer.

Fixes #19

messages/SLGetGatewayDataMessage.js

@@ -7,7 +7,7 @@ const MSG_ID = 18003;
 exports.SLGetGatewayDataMessage = class SLGetGatewayDataMessage extends SLMessage {
   constructor(buf) {
     var size;
-    if (buf) {
+    if (buf && typeof buf === 'object') {
       size = buf.readInt32LE(4) + 8;
     }
     super(0, MSG_ID, size);