merge revision(s) 43334: [Backport #9030]

	* vm_insnhelper.c (vm_call_method): set ci->me to 0 when the
	  original method of a refined method is undef to avoid SEGV.

	* vm_method.c (rb_method_entry_without_refinements): return 0 when
	  the original method of a refined method is undef to avoid SEGV.

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


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@43345 b2dd03c8-39d4-4d8f-98ff-823fe69b080e