shutdown() can raise an "empty" exception

Where by "empty" I mean:

ipdb> err
Error([],)
ipdb> err.args
([],)
ipdb> err.message
[]

This comes from a user who reported a bug on pyftpdlib issue tracker:
http://code.google.com/p/pyftpdlib/issues/detail?id=171

The problem appears to be here:

    MY_BEGIN_ALLOW_THREADS(self->tstate)
    ret = SSL_shutdown(self->ssl);
    MY_END_ALLOW_THREADS(self->tstate)

    if (PyErr_Occurred())
    {
        flush_error_queue();
        return NULL;
    }

The right way to handle this, IMO, would be to avoid calling PyErr_Occurred() / flush_error_queue() at all and check SSL_shutdown return code and then call SSL_get_error() instead.

If SSL_get_error() returns SSL_ERROR_NONE (as in this case) it would make sense to just return Py_False.
For the upper application this means that Connection.shutdown() must be called again in order the complete the shutdown process (note: I'm using non blocking sockets).