NPE when closing WebSocket

2.1.0-beta7:

```
java.lang.NullPointerException: null
        at org.asynchttpclient.netty.ws.NettyWebSocket.releaseBufferedFrames(NettyWebSocket.java:84)
        at org.asynchttpclient.netty.ws.NettyWebSocket.close(NettyWebSocket.java:252)
        at org.asynchttpclient.netty.handler.WebSocketHandler.handleChannelInactive(WebSocketHandler.java:163)
```