Fix possible race condition in websocket heartbeat callback

Author: Dan Ordille

src/trans-websocket.coffee

@@ -101,7 +101,8 @@ class WebSocketReceiver extends transport.GenericReceiver
             super
 
     heartbeat_timeout: ->
-        @session.close(3000, 'No response from heartbeat')
+        if @session?
+            @session.close(3000, 'No response from heartbeat')