vm_backtrace.c: unknown name method

* vm_backtrace.c (location_format): adjust unknown name method
  results to other functions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47950 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
vm_backtrace.c: fix unknown ID name

* vm_backtrace.c (id2str): adapt rb_id2str which returns 0 on
  unknown ID to return Qnil, since oldbt_arg functions expect Qnil
  on unknown method names.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47951 b2dd03c8-39d4-4d8f-98ff-823fe69b080e