fix turnoff: repeater is receptor AND effector

mesecons/internal.lua

@@ -569,14 +569,16 @@ function mesecon.turnoff(pos, link)
 							--return false
 						end
 					end
-				elseif mesecon.is_effector(n.name) then
+				end
+				if n and mesecon.is_effector(n.name) then
 					insert(signals, {
 						pos = np,
 						node = n,
 						link = r,
 						depth = depth
 					})
-				elseif mesecon.is_conductor_on(n) and f.link.opaquespread then
+				end
+				if mesecon.is_conductor_on(n) and f.link.opaquespread then
 					insert(frontiers, {pos = np, link = r})
 				end
 				for _, l in pairs(mesecon.rules_link_rule_all(fpos, r)) do