http: fix "Cannot call method 'emit' of null"

Fix the following exception:

  http.js:974
    this._httpMessage.emit('close');
                      ^
  TypeError: Cannot call method 'emit' of null
      at Socket.onServerResponseClose (http.js:974:21)
      at Socket.EventEmitter.emit (events.js:124:20)
      at net.js:421:10
      at process._tickCallback (node.js:386:13)
      at process._makeCallback (node.js:304:15)

Fixes #4586.