Ensure this._transport is valid in debug msg in _open

Fix #448

Author: brycekahle

lib/main.js

@@ -309,7 +309,7 @@ SockJS.prototype._transportClose = function(code, reason) {
 };
 
 SockJS.prototype._open = function() {
-  debug('_open', this._transport.transportName, this.readyState);
+  debug('_open', this._transport && this._transport.transportName, this.readyState);
   if (this.readyState === SockJS.CONNECTING) {
     if (this._transportTimeoutId) {
       clearTimeout(this._transportTimeoutId);