evdev: Use touch ID 0 when generating touch up event

device->mt.slot is uninitialized when we're not receiving the
evdev slot events.  Always use ID 0 as we do when we generate the
touch down and motion events.

src/evdev.c

@@ -86,7 +86,7 @@ evdev_process_key(struct evdev_device *device, struct input_event *e, int time)
 
 	case BTN_TOUCH:
 		if (e->value == 0 && !device->is_mt)
-			notify_touch(device->seat, time, device->mt.slot, 0, 0,
+			notify_touch(device->seat, time, 0, 0, 0,
 				     WL_TOUCH_UP);
 		break;
 	default: