Use get_auth_handler().get_auth() instead of auth_table

minetest.auth_table is an implementation detail of the default auth handler.
No guarantee is made that it even exists and using this table directly is incompatible
with custom auth handlers.  Instead, use the proper auth handler API.

internal.lua

@@ -1,6 +1,6 @@
 
 function areas:player_exists(name)
-	return minetest.auth_table[name] ~= nil
+	return minetest.get_auth_handler().get_auth(name) ~= nil
 end
 
 -- Save the areas table to a file