Prevent server from leaking error handler.

Revealed in warning:

```
(node) warning: possible EventEmitter memory leak detected. 11 listeners added. Use emitter.setMaxListeners() to increase limit.
Trace
    at Server.addListener (events.js:160:15)
    at servertest (/*/*/*/node_modules/servertest/servertest.js:72:6)
```

Could increase listener limit, and it's currently safe because callback is nooped after 1st calling, but the listener probably shouldn't be there regardless.