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

1 files changed, 12 insertions, 0 deletions

diff --git a/test/ruby/test_regexp.rb b/test/ruby/test_regexp.rb
index f22128686e..be7c6761ca 100644
--- a/test/ruby/test_regexp.rb
+++ b/test/ruby/test_regexp.rb
@@ -999,6 +999,18 @@ class TestRegexp < Test::Unit::TestCase
     assert_equal('foobazquux/foobazquux', result, bug8856)
   end
 
+  def test_regsub_no_memory_leak
+    assert_no_memory_leak([], "#{<<~"begin;"}", "#{<<~"end;"}", rss: true)
+      code = proc do
+        "aaaaaaaaaaa".gsub(/a/, "")
+      end
+
+      1_000.times(&code)
+    begin;
+      100_000.times(&code)
+    end;
+  end
+
   def test_ignorecase
     v = assert_deprecated_warning(/variable \$= is no longer effective/) { $= }
     assert_equal(false, v)