delegate.rb: check if target is set

* lib/delegate.rb (SimpleDelegator#__getobj__): target object must be set.
* lib/delegate.rb (DelegateClass#__getobj__): ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43759 b2dd03c8-39d4-4d8f-98ff-823fe69b080e