Fix leak of AST when Ripper#compile_error jumps

For example, the following script leaks:

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

      def compile_error(msg) = @blk.call(msg)
    end

    def call_parse = MyRipper.new("/") { |msg| return msg }.parse

    10.times do
      100_000.times do
        call_parse
      end

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

Before:

    93952
    169040
    244224
    318784
    394432
    468224
    544048
    618560
    693776
    768384

After:

    19776
    19776
    20352
    20880
    20912
    21408
    21328
    21152
    21472
    20944

0 files changed, 0 insertions, 0 deletions