TestPKeyDH#test_DEFAULT_parameters dumps core under GC stress

[k-tsj]

Steps to reproduce:

$ cd PATH_TO_RUBY_REPOS
$ make TESTS='--gc-stress -n /test_DEFAULT_parameters/ openssl/test_pkey_dh.rb' test-all
(snip)
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.2) 5.4.0 20160609
(snip)
[1/1] OpenSSL::TestPKeyDH#test_DEFAULT_parameters/home/ktsj/work/ruby/test/openssl/test_pkey_dh.rb:20: [BUG] Segmentation fault at 0x00000000000000
ruby 2.4.0dev (2016-12-03 master 56964) [x86_64-linux]

I have attached backtrace log.

[rhenium]

Thanks for reporting. 182604c (merged into 'maint') should fix the bug.