Fix memory leak in syntax error in prism

If there is a syntax error, there could be an ast_node in the result.
This could get leaked if there is a syntax error so parsing could not
complete (parsed is not set to true).

For example, the following script leaks memory:

    10.times do
      10_000.times do
        eval("def foo(...) super(...) {}; end")
      rescue SyntaxError
      end

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

Before:

    31328
    42768
    53856
    65120
    76208
    86768
    97856
    109120
    120208
    131296

After:

    20944
    20944
    20944
    20944
    20944
    20944
    20944
    20944
    20944
    20944

0 files changed, 0 insertions, 0 deletions