SystemError raised when access to `listen_queue` method of uwsgi in FreeBSD

When call `uwsgi.listen_queue()` in FreeBSD, it raised:

<pre>
SystemError: error return without exception set
</pre>


sample code:

``` python
#! /usr/bin/env python
import uwsgi

def application(env, start_response):
    start_response('200 OK', [('Content-Type','text/html')])
    listen_queue = uwsgi.listen_queue()
    return "listen queue: {0}".format(listen_queue)

```
