[MIRROR] Fixes runtime in z level update (#2224) (#3098)

* Fixes runtime in z level update (#82898)

## About The Pull Request

re-adds the check that they are actually being moved to a new z level in
the update z level proc so it doesn't check null z level for cliented
mobs and ai controllers.

## Why It's Good For The Game

Saw this runtime on moth.fans and thought i should fix it.

## Changelog

Nothing player-facing.

* Fixes runtime in z level update

---------

Co-authored-by: NovaBot <[email protected]>
Co-authored-by: John Willard <[email protected]>

code/modules/mob/living/living.dm

@@ -1855,25 +1855,27 @@ GLOBAL_LIST_EMPTY(fire_appearances)
 		return
 
 	//Check the amount of clients exists on the Z level we're leaving from,
-	//this excludes us as we haven't added ourselves to the new z level yet.
+	//this excludes us because at this point we are not registered to any z level.
 	var/old_level_new_clients = (registered_z ? SSmobs.clients_by_zlevel[registered_z].len : null)
 	//No one is left after we're gone, shut off inactive ones
 	if(registered_z && old_level_new_clients == 0)
 		for(var/datum/ai_controller/controller as anything in SSai_controllers.ai_controllers_by_zlevel[registered_z])
 			controller.set_ai_status(AI_STATUS_OFF)
 
-	//Check the amount of clients exists on the Z level we're moving towards, excluding ourselves.
-	var/new_level_old_clients = SSmobs.clients_by_zlevel[new_z].len
+	if(new_z)
+		//Check the amount of clients exists on the Z level we're moving towards, excluding ourselves.
+		var/new_level_old_clients = SSmobs.clients_by_zlevel[new_z].len
+
+		//We'll add ourselves to the list now so get_expected_ai_status() will know we're on the z level.
+		SSmobs.clients_by_zlevel[new_z] += src
+
+		if(new_level_old_clients == 0) //No one was here before, wake up all the AIs.
+			for (var/datum/ai_controller/controller as anything in SSai_controllers.ai_controllers_by_zlevel[new_z])
+				//We don't set them directly on, for instances like AIs acting while dead and other cases that may exist in the future.
+				//This isn't a problem for AIs with a client since the client will prevent this from being called anyway.
+				controller.set_ai_status(controller.get_expected_ai_status())
 
 	registered_z = new_z
-	//We'll add ourselves to the list now so get_expected_ai_status() will know we're on the z level.
-	SSmobs.clients_by_zlevel[registered_z] += src
-
-	if(new_level_old_clients == 0) //No one was here before, wake up all the AIs.
-		for (var/datum/ai_controller/controller as anything in SSai_controllers.ai_controllers_by_zlevel[new_z])
-			//We don't set them directly on, for instances like AIs acting while dead and other cases that may exist in the future.
-			//This isn't a problem for AIs with a client since the client will prevent this from being called anyway.
-			controller.set_ai_status(controller.get_expected_ai_status())
 
 /mob/living/on_changed_z_level(turf/old_turf, turf/new_turf, same_z_layer, notify_contents)
 	..()