iseq.c (iseq_data_to_ary): keep hidden variables

We must not ignore hidden variables when rb_id2str fails.
Thanks to wanabe [ruby-core:66566]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48678 b2dd03c8-39d4-4d8f-98ff-823fe69b080e