ruby.c: do not modify shared string

* ruby.c (ruby_init_loadpath_safe): get rid of modify shared
  string directly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48742 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-12-09

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48743 b2dd03c8-39d4-4d8f-98ff-823fe69b080e