Fixes division by zero (tgstation#50800)

code/modules/surgery/organs/lungs.dm

@@ -29,6 +29,7 @@
 	var/safe_co2_min = 0
 	var/safe_co2_max = 10 // Yes it's an arbitrary value who cares?
 	var/safe_toxins_min = 0
+	///How much breath partial pressure is a safe amount of toxins. 0 means that we are immune to toxins.
 	var/safe_toxins_max = 0.05
 	var/SA_para_min = 1 //Sleeping agent
 	var/SA_sleep_min = 5 //Sleeping agent
@@ -248,7 +249,7 @@
 				SEND_SIGNAL(owner, COMSIG_ADD_MOOD_EVENT, "chemical_euphoria", /datum/mood_event/chemical_euphoria)
 		else
 			SEND_SIGNAL(owner, COMSIG_CLEAR_MOOD_EVENT, "chemical_euphoria")
-		if(SA_pp > safe_toxins_max*3)
+		if(safe_toxins_max && SA_pp > safe_toxins_max*3)
 			var/ratio = (breath_gases[/datum/gas/nitrous_oxide][MOLES]/safe_toxins_max)
 			H.apply_damage_type(clamp(ratio, tox_breath_dam_min, tox_breath_dam_max), tox_damage_type)
 			H.throw_alert("too_much_tox", /obj/screen/alert/too_much_tox)