vm_method.c: fix infinite recursion

* vm_method.c (rb_method_entry_get_without_cache): get rid of
  infinite recursion at aliases in a subclass and a superclass.
  return actually defined class for other than singleton class.
  [ruby-core:60431] [Bug #9475]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45367 b2dd03c8-39d4-4d8f-98ff-823fe69b080e