class.c: do not freeze class of hidden object

* class.c (rb_freeze_singleton_class): get rid of freeze class of
  hidden object to fix segfaults.

* include/ruby/ruby.h (rb_obj_freeze_inline): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47637 b2dd03c8-39d4-4d8f-98ff-823fe69b080e