test_pair.rb: thread leak

* test/openssl/test_pair.rb (test_ecdh_callback): fix thread leak.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51433 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/openssl/test_pair.rb

@@ -368,6 +368,7 @@ module OpenSSL::TestPairM
     sock1.close if sock1
     sock2.close if sock2
     accepted.close if accepted.respond_to?(:close)
+    th.join rescue nil
   end
 
   def test_connect_accept_nonblock_no_exception