Added code protecting against obscure merges of invalid zones.

- Usually happened when welding fuel tanks caused secondary explosions.

code/ZAS/ConnectionGroup.dm

@@ -143,6 +143,9 @@ Class Procs:
 	. = ..()
 
 /connection_edge/zone/tick()
+	if(A.invalid || B.invalid)
+		erase()
+		return
 	//world << "[id]: Tick [air_master.current_cycle]: \..."
 	if(direct)
 		if(air_master.equivalent_pressure(A, B))
@@ -207,6 +210,9 @@ Class Procs:
 	return A == Z
 
 /connection_edge/unsimulated/tick()
+	if(A.invalid)
+		erase()
+		return
 	//world << "[id]: Tick [air_master.current_cycle]: To [B]!"
 	//A.air.mimic(B, coefficient)
 	ShareSpace(A.air,air)

code/ZAS/Controller.dm

@@ -220,6 +220,8 @@ Total Unsimulated Turfs: [world.maxx*world.maxy*world.maxz - simulated_turf_coun
 	#ifdef ZASDBG
 	ASSERT(istype(A))
 	ASSERT(istype(B))
+	ASSERT(!A.invalid)
+	ASSERT(!B.invalid)
 	ASSERT(A != B)
 	#endif
 	if(A.contents.len < B.contents.len)
@@ -234,6 +236,7 @@ Total Unsimulated Turfs: [world.maxx*world.maxy*world.maxz - simulated_turf_coun
 	ASSERT(istype(A))
 	ASSERT(isturf(B))
 	ASSERT(A.zone)
+	ASSERT(!A.zone.invalid)
 	//ASSERT(B.zone)
 	ASSERT(A != B)
 	#endif