Fix crash when doors.registered_doors isn't available

mesecons_doors/init.lua

@@ -132,6 +132,7 @@ local function meseconify_trapdoor(name)
 end
 
 minetest.register_on_mods_loaded(function()
+	if doors.registered_doors ~= nil then -- registered_doors is not always available
 		for k,_ in pairs(doors.registered_doors) do
 			if k:find("_a$") then
 				meseconify_door(k:sub(1,-3))
@@ -142,4 +143,5 @@ minetest.register_on_mods_loaded(function()
 				meseconify_trapdoor(k)
 			end
 		end
+	end
 end)