Fix memory leak in rb_reg_search_set_match

https://github.com/ruby/ruby/pull/12801 changed regexp matches to reuse
the backref, which causes memory to leak if the original registers of the
match is not freed.

For example, the following script leaks memory:

    10.times do
      1_000_000.times do
        "aaaaaaaaaaa".gsub(/a/, "")
      end

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

Before:

    774256
    1535152
    2297360
    3059280
    3821296
    4583552
    5160304
    5091456
    5114256
    4980192

After:

    12480
    11440
    11696
    11632
    11632
    11760
    11824
    11824
    11824
    11888

0 files changed, 0 insertions, 0 deletions