Memory leak when duplicating identhash

[Bug #20145]

Before this commit, both copy_compare_by_id and hash_copy will create a
copy of the ST table, so the ST table created in copy_compare_by_id will
be leaked.

    h = { 1 => 2 }.compare_by_identity

    10.times do
      1_000_000.times do
        h.select { false }
      end

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

Before:

    110736
    204352
    300272
    395520
    460704
    476736
    542000
    604704
    682624
    770528

After:

    15504
    16048
    16144
    16256
    16320
    16320
    16752
    16752
    16752
    16752

0 files changed, 0 insertions, 0 deletions