object.c: avoid inadvertent symbol creation

* object.c (rb_mod_const_set): fix symbol name check.
  (rb_obj_ivar_set): ditto.
  (rb_mod_cvar_set): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40104 b2dd03c8-39d4-4d8f-98ff-823fe69b080e