parse.y: do not use disposed string

* parse.y (parser_here_document): do not append already appended
  and disposed code fragment.  [ruby-dev:48647] [Bug #10392]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47973 b2dd03c8-39d4-4d8f-98ff-823fe69b080e