ossl_pkey.c: fix memory leak

* ext/openssl/ossl_pkey.c (ossl_pkey_verify): as EVP_VerifyFinal()
  finalizes only a copy of the digest context, the context must be
  cleaned up after initialization by EVP_MD_CTX_cleanup() or a
  memory leak will occur.  [ruby-core:62038] [Bug #9743]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45595 b2dd03c8-39d4-4d8f-98ff-823fe69b080e