Fix memory leak in Ripper.sexpHEADmaster

rb_ast_dispose does not free the rb_ast_t causing it to be leaked. This
commit changes it to use rb_ast_free instead.

For example:

    require "ripper"

    10.times do
      100_000.times do
        Ripper.sexp("")
      end

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

Before:

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After:

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    22784
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    22912
    22912
    22912
    22912

0 files changed, 0 insertions, 0 deletions