vm_method.c: no redefinition warnings for undefined methods

* vm_method.c (rb_method_entry_make): warn redefinition only for
  already defined methods, but not for undefined methods.
  [ruby-dev:48691] [Bug #10421]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48111 b2dd03c8-39d4-4d8f-98ff-823fe69b080e