merge revision(s) 43090,43091: [Backport #8966]

	* 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.
	  of a refined method is not defined.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@43143 b2dd03c8-39d4-4d8f-98ff-823fe69b080e