Fix memory leak of rb_ast_t in parser

ast_alloc uses TypedData_Make_Struct, which allocates a rb_ast_t. But it
is overwritten when we set the DATA_PTR so the original memory is leaked.

For example:

    10.times do
      100_000.times do
        eval("")
      end

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

Before:

    17328
    20752
    23664
    28400
    30656
    34224
    37424
    40784
    43328
    46656

After:

    14320
    14320
    14320
    14320
    14320
    14320
    14320
    14336
    14336
    14336

0 files changed, 0 insertions, 0 deletions