Sending a zero-length UDP packet to asyncore closes socket by default.

The default implementation of recv() assumes that zero-length data means close.  This isn't true for UDP - it is possible to send a zero-length payload packet.

A possible work-around is to overload handle_close() and not call self.close(). However, as handle_close() is called as part of exception handling, we can't determine whether the handle_close() is happening due to a zero-length payload or an exception event.

It should be possible to process a zero-length payload UDP packet.