class.c: check redefinition

* class.c (rb_prepend_module): check redefinition of built-in opimized
  methods.  [ruby-dev:47124] [Bug #7983]
* vm.c (rb_vm_check_redefinition_by_prepend): ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39601 b2dd03c8-39d4-4d8f-98ff-823fe69b080e