merge revision(s) 55427: [Backport ]

    * string.c (tr_trans): consider terminator length and fix heap
      overflow.  reported by Guido Vranken <guido AT guidovranken.nl>.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@55872 b2dd03c8-39d4-4d8f-98ff-823fe69b080e