libinput: Don't process touch events for devices without a valid output

That would be the case of a touch screen mapped to an output that was
unplugged.
diff --git a/src/libinput-device.c b/src/libinput-device.c
index 0ca6c4b..a67c119 100644
--- a/src/libinput-device.c
+++ b/src/libinput-device.c
@@ -147,6 +147,9 @@
 	uint32_t time;
 	int32_t slot;
 
+	if (!device->output)
+		return;
+
 	time = libinput_event_touch_get_time(touch_event);
 	slot = libinput_event_touch_get_seat_slot(touch_event);