timer: fix integer overflow in timer_delay()

Fixes a regression: for output_plugin.delay(), we added a method to
the timer class which returns the delay in milliseconds.  This fails
to detect negative values, because the unsigned integer is divided by
1000, and then casted to signed.

src/timer.c

@@ -74,7 +74,7 @@ void timer_add(Timer *timer, int size)
 unsigned
 timer_delay(const Timer *timer)
 {
-	int64_t delay = (timer->time - now()) / 1000;
+	int64_t delay = (int64_t)(timer->time - now()) / 1000;
 	if (delay < 0)
 		return 0;