Picked from upstream: https://github.com/ruby/psych/commit/66fb7fb77533d0606748940288858794752d95cb
[ci skip][ruby-dev:49879][Bug #12942]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56806 b2dd03c8-39d4-4d8f-98ff-823fe69b080e