timer: if a timer is inactive, do not call the timer func

Race conditions may happen where code that cancels a timer is called just
as that timer triggers. If we cancel a timer, we assume that we've put the
code into a state where the timer firing will trigger a bug.

This could be observed with the middle button code if the release event was
held back just long enough. The button release code cancelled the timer, set
the state back to idle and then complained when the timeout handling sent a
'timeout' event while being in idle.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 696fdff247)

src/timer.c

@@ -133,6 +133,9 @@ libinput_timer_handler(void *data)
 		return;
 
 	list_for_each_safe(timer, tmp, &libinput->timer.list, link) {
+		if (timer->expire == 0)
+			continue;
+
 		if (timer->expire <= now) {
 			/* Clear the timer before calling timer_func,
 			   as timer_func may re-arm it */