rdoc/constant.rb: workaround of NoMethodError

* lib/rdoc/constant.rb (RDoc::Constant#documented?): workaround for
  NoMethodError when the original of alias is not found.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43006 b2dd03c8-39d4-4d8f-98ff-823fe69b080e