http: incoming parser set to null twice causes crash

The error:

```
http.js:423
  this.socket.parser.incoming = null;
                              ^
TypeError: Cannot set property 'incoming' of null
    at IncomingMessage._dump (http.js:423:31)
    at ServerResponse.<anonymous> (http.js:1979:13)
    at ServerResponse.EventEmitter.emit (events.js:91:17)
    at ServerResponse.OutgoingMessage._finish (http.js:963:8)
    at ServerResponse.OutgoingMessage.end (http.js:946:10)
    at IncomingMessage.<anonymous> (###### user code #####)
    at IncomingMessage.EventEmitter.emit (events.js:116:20)
    at _stream_readable.js:852:12
    at process._tickCallback (node.js:403:13)
```