Fix leak in warning of duplicate keys when Ripper#warn jumps

For example, the following code leaks:

    class MyRipper < Ripper
      def initialize(src, &blk)
        super(src)
        @blk = blk
      end

      def warn(msg, *args) = @blk.call(msg)
    end

    $VERBOSE = true
    def call_parse = MyRipper.new("if true\n  end\n") { |msg| return msg }.parse

    10.times do
      500_000.times do
        call_parse
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    34832
    51952
    69760
    88048
    105344
    123040
    141152
    159152
    176656
    194272

After:

    18400
    20256
    20272
    20272
    20272
    20304
    20368
    20368
    20368
    20400

0 files changed, 0 insertions, 0 deletions