Port of @PsiOmegaDelta's Baystation12/Baystation12#16820

Ports tg's garbage collector subsystem and Destroy() returning qdel hints.

code/game/objects/items/devices/transfer_valve.dm

@@ -169,7 +169,7 @@
 	
 	valve_open = 0
 	
-	if(deleted(tank_one) || deleted(tank_two))
+	if(QDELETED(tank_one) || QDELETED(tank_two))
 		return
 	
 	var/ratio1 = tank_one.air_contents.volume/tank_two.air_contents.volume

code/game/objects/items/weapons/flamethrower.dm

@@ -27,15 +27,10 @@
 
 
 /obj/item/weapon/flamethrower/Destroy()
-	if(weldtool)
-		qdel(weldtool)
-	if(igniter)
-		qdel(igniter)
-	if(ptank)
-		qdel(ptank)
-	..()
-	return
-
+	qdel_null(weldtool)
+	qdel_null(igniter)
+	qdel_null(ptank)
+	. = ..()
 
 /obj/item/weapon/flamethrower/process()
 	if(!lit)

code/game/objects/items/weapons/material/material_weapons.dm

@@ -71,7 +71,7 @@
 
 /obj/item/weapon/material/Destroy()
 	processing_objects -= src
-	..()
+	. = ..()
 
 /obj/item/weapon/material/apply_hit_effect()
 	..()