* vm_method.c (rb_undef): raise a NameError if the original method
  of a refined method is not defined.

* vm_insnhelper.c (rb_method_entry_eq): added NULL check to avoid SEGV.

* test/ruby/test_refinement.rb: related test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43090 b2dd03c8-39d4-4d8f-98ff-823fe69b080e