Don't blow up when using nested fields_for and the source object is nil

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@7168 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

actionpack/lib/action_view/helpers/form_helper.rb

@@ -483,7 +483,7 @@ def to_content_tag(tag_name, options = {})
       end
       
       def object
-        @object || @template_object.instance_variable_get("@#{@object_name}")
+        @object || (@template_object.instance_variable_get("@#{@object_name}") rescue nil)
       end
 
       def value(object)