[MIRROR] Fix two gas assertion runtimes (#816)

* Fix two gas assertion runtimes (#53718)

/datum/gas_mixture/proc/remove(amount) has a code path that can return null.

All the code following the runtimes assume some bit of gas_mixture was actually removed at all. No need to trigger this code if there's no gas_mixture removed, can early return instead.

* Fix two gas assertion runtimes

Co-authored-by: Timberpoes <silent_insomnia_pp@hotmail.co.uk>

code/modules/events/crystal_event.dm

@@ -363,6 +363,10 @@ This section is for the destabilized SM
 	var/datum/gas_mixture/removed
 	var/gasefficency = 0.5
 	removed = env.remove(gasefficency * env.total_moles())
+
+	if(!removed)
+		return
+
 	removed.assert_gases(/datum/gas/bz, /datum/gas/miasma)
 	if(!removed || !removed.total_moles() || isspaceturf(loc_turf))
 		removed.gases[/datum/gas/bz][MOLES] += 0.5