Fixes some runtimes with soapstone messages

I'm not sure why they're not loading on main anymore, possibly some bad
JSON data? This stopped the warning locally, anyway.

code/modules/library/soapstone.dm

@@ -192,10 +192,14 @@
 	var/turf/T = get_turf(src)
 	data["x"] = T.x
 	data["y"] = T.y
+	data["z"] = T.z
 	data["like_keys"] = like_keys
 	data["dislike_keys"] = dislike_keys
 
 /obj/structure/chisel_message/proc/unpack(list/data)
+	if(!islist(data))
+		return
+
 	hidden_message = data["hidden_message"]
 	creator_name = data["creator_name"]
 	creator_key = data["creator_key"]
@@ -209,8 +213,10 @@
 
 	var/x = data["x"]
 	var/y = data["y"]
-	var/turf/newloc = locate(x, y, ZLEVEL_STATION)
-	forceMove(newloc)
+	var/z = data["z"]
+	var/turf/newloc = locate(x, y, z)
+	if(isturf(newloc))
+		forceMove(newloc)
 	update_icon()
 
 /obj/structure/chisel_message/examine(mob/user)