Fix warnings about accessing unitialized variable

in ad member service

lib/ldap_fluff/ad_member_service.rb:29: warning: instance variable
@domain_functionality not initialized

lib/ldap_fluff/ad_member_service.rb

@@ -26,7 +26,7 @@ def find_user_groups(uid)
 
   # return the domain functionality level, default to 0
   def _get_domain_func_level
-    return @domain_functionality unless @domain_functionality.nil?
+    return @domain_functionality if defined?(@domain_functionality)
 
     @domain_functionality = 0