avahi_simple_poll_iterate does not check for new events when sleep_time is zero

[armin]

The documentation states "If sleep_time is 0 the routine just checks if any event is pending." The code however does not even call the poll function in avahi_simple_poll_run when the prepared timeout is zero (which is for example always the case with a zero sleep time and no further timeouts).

[lennart]

(In [1425]) Remove an invalid optimization in avahi_simple_poll_run(). (Closes #132)