Fix for crit bug when closing pipe stream

When we use pipe method of BinaryStream and close connected browser page - server got crit error:

```js
TypeError: Cannot call method '_onClose' of undefined
  at WebSocket.<anonymous> (/srv/http/socialstream/server/node_modules/binaryjs/lib/client.js:59:28)
  at WebSocket.EventEmitter.emit (events.js:117:20)
  at WebSocket.cleanupWebsocketResources (/srv/http/socialstream/server/node_modules/binaryjs/node_modules/streamws/lib/WebSocket.js:619:23)
  at Socket.EventEmitter.emit (events.js:117:20)
  at _stream_readable.js:920:16
  at process._tickCallback (node.js:415:13)
```

It is fast fix for this. It easy to found reason by following callstack - when stream is closed it automatically close his pipe destination stream and removed it from self.streams array.
I don't think we need this check since we're just iterating over stream ids.