leds: Fix trigger unregister_simple if register_simple fails

Fix led_trigger_unregister_simple to handle the case where
led_trigger_register_simple fails, avoiding a NULL pointer
dereference.
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>

drivers/leds/led-triggers.c

@@ -215,7 +215,8 @@ void led_trigger_unregister(struct led_trigger *trigger)
 
 void led_trigger_unregister_simple(struct led_trigger *trigger)
 {
-	led_trigger_unregister(trigger);
+	if (trigger)
+		led_trigger_unregister(trigger);
 	kfree(trigger);
 }