ruby.c: do not modify shared string
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48742 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
ruby.c: do not modify shared string
string directly.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48742 b2dd03c8-39d4-4d8f-98ff-823fe69b080e