Fix memory leak in the parser

[peterzhu2118]

Reproduction script:

require "ripper"

10.times do
  20_000.times do
    Ripper.parse("")
  end

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

Before:

28032
34432
40704
47232
53632
60032
66432
72832
79232
85632

After:

21760
21760
21760
21760
21760
21760
21760
21760
21760
21760