Fix occasional error with sending while readyState !== 1

Under certain conditions, noVNC would attempt to flush the web socket
while it was disconnected, before the disconnected state was picked up.
This casues noVNC to crash ungracefully and the parent window is not
notified - leading to no chance at recovery without a page refresh.

include/websock.js

@@ -197,7 +197,7 @@ function Websock() {
             }
 
             if (this._websocket.bufferedAmount < this.maxBufferedAmount) {
-                if (this._sQlen > 0) {
+                if (this._sQlen > 0 && this._websocket.readyState === WebSocket.OPEN) {
                     this._websocket.send(this._encode_message());
                     this._sQlen = 0;
                 }