Flaky repro for compaction issue

ossl_ssl_initialize() sets a Ruby reference on an OpenSSL handle and
it's retrived later for calling callbacks such as session_new_cb. There
is nothing marking the reference that lives on the SSL handle, so the
reference could move and cause crashes later when the stale reference is
used.

On my machine I get a crash with this change running the following tests
in a loop:

    bundle exec rake test TEST=test/openssl/test_ssl_session.rb TESTOPTS='-n/cb/'

test/openssl/test_ssl_session.rb

@@ -387,7 +387,12 @@ def test_dup
  def server_connect_with_session(port, ctx = nil, sess = nil)
  sock = TCPSocket.new("127.0.0.1", port)
  ctx ||= OpenSSL::SSL::SSLContext.new
- ssl = OpenSSL::SSL::SSLSocket.new(sock, ctx)
+ @ssl = OpenSSL::SSL::SSLSocket.new(sock, ctx)
+ itself
+ if GC.respond_to?(:verify_compaction_references)
+ GC.verify_compaction_references(double_heap: true, toward: :empty)
+ end
+ ssl = @ssl
  ssl.session = sess if sess
  ssl.sync_close = true
  ssl.connect