Fix memory leak in prism when syntax error in iseq compilation

If there's a syntax error during iseq compilation then prism would leak
memory because it would not free the pm_parse_result_t.

This commit changes pm_iseq_new_with_opt to have a rb_protect to catch
when an error is raised, and return NULL and set error_state to a value
that can be raised by calling rb_jump_tag after memory has been freed.

For example:

    10.times do
      10_000.times do
        eval("/[/=~s")
      rescue SyntaxError
      end

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

Before:

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    275376
    304592

After:

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    14256
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    16000
    16064
    17232
    17952

0 files changed, 0 insertions, 0 deletions