[PRISM] Fix memory leak in iseq

rb_iseq_compile_prism_node calls both rb_translate_prism and iseq_setup.
Both of these functions call iseq_set_sequence. This means that the first
iseq_set_sequence will leak because the iseq will be overwritten.

For example:

    10.times do
      100_000.times do
        RubyVM::InstructionSequence.compile_prism("")
      end

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

Before:

    20528
    27328
    33840
    40208
    46400
    52960
    59168
    65600
    71888
    78352

After:

    13696
    13712
    13712
    13712
    13712
    14352
    14352
    14992
    14992
    14992

0 files changed, 0 insertions, 0 deletions