technic_cnc/cnc.lua: Fix access to non-existant stack parameter (#528)

technic_cnc/cnc.lua

@@ -59,7 +59,7 @@ else
 		if minetest.is_protected(pos, player:get_player_name()) then
 			return 0
 		end
-		return stack:get_count()
+		return count
 	end
 
 	can_dig = function(pos, player)