merge revision(s) 66111: [Backport #14561]
Avoid GCing dead stack after switching away from a fiber Fixes <https://bugs.ruby-lang.org/issues/14561> and discussed <https://bugs.ruby-lang.org/issues/15362>.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@66777 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merge revision(s) 66111: [Backport #14561]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@66777 b2dd03c8-39d4-4d8f-98ff-823fe69b080e