The only usage of rb_fiber_reset_root_local_storage() is from
ruby_vm_destruct(), where the object space is already terminated.
This `th->self` is not alive.  Why not just use `th` itself.

See also: https://travis-ci.org/ruby/ruby/jobs/451294954


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65574 b2dd03c8-39d4-4d8f-98ff-823fe69b080e