Fix: r32043 may raise NameError.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@32048 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/syck/lib/syck/rubytypes.rb

@@ -10,7 +10,7 @@ end
 class Object
     yaml_as "tag:ruby.yaml.org,2002:object"
     def to_yaml_style; end
-    undef to_yaml_properties
+    undef to_yaml_properties rescue nil
     def to_yaml_properties; instance_variables.sort; end
 	def to_yaml( opts = {} )
 		YAML::quick_emit( self, opts ) do |out|