Description:
------------
If the priority queue of scheduled events is empty, the dispatch loop should terminate successfully. This bug might be related to the fact that event_have_events returns true even if there are no events. 

Reproduce code:
---------------
<?php

dl('event.so');

event_init();

// called when user hits ^C
function stop($sig) {
    global $s;
    echo "You got signal $sig. Descheduling event.\n";
    event_deschedule($s);
}

$s = event_handle_signal(2, 'stop');

event_schedule($s);

echo "Going into dispatch loop: at t= " . time() . "\n";
event_dispatch();

echo "all done\n";

?>


Expected result:
----------------
all done

program exits.

Actual result:
--------------
program remains stuck in the dispatch loop. "all done" is never reached.