* object.c (rb_mod_cvar_set): call to_str for string only once.
  to_str was called from rb_is_class_name and rb_to_id before.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40174 b2dd03c8-39d4-4d8f-98ff-823fe69b080e