Makes the mining asteroid airless again.

This turf surrounds the edges of the map.

air was "leaking" through it from off the map filling the zones that touch it with air.

fixes: #11365

code/modules/mining/mine_turfs.dm

@@ -3,6 +3,7 @@
 	name = "impassable rock"
 	icon = 'icons/turf/walls.dmi'
 	icon_state = "rock-dark"
+	blocks_air = 1
 
 /turf/simulated/mineral //wall piece
 	name = "Rock"
@@ -525,4 +526,4 @@
 			else if(istype(R.module_state_3,/obj/item/weapon/storage/bag/ore))
 				attackby(R.module_state_3,R)
 			else
-				return
+				return