* ext/openssl/ossl_x509cert.c (ossl_x509_verify): X509_verify()
  family may put errors on 0 return (0 means verification failure).
  Clear OpenSSL error queue before return to Ruby. Since the queue is
  thread global, remaining errors in the queue can cause an unexpected
  error in the next OpenSSL operation.  [ruby-core:48284] [Bug #7215]

* ext/openssl/ossl_x509crl.c (ossl_x509crl_verify): ditto.

* ext/openssl/ossl_x509req.c (ossl_x509req_verify): ditto.

* ext/openssl/ossl_x509store.c (ossl_x509stctx_verify): ditto.

* ext/openssl/ossl_pkey_dh.c (dh_generate): clear the OpenSSL error
  queue before re-raising exception.

* ext/openssl/ossl_pkey_dsa.c (dsa_generate): ditto.

* ext/openssl/ossl_pkey_rsa.c (rsa_generate): ditto.

* ext/openssl/ossl_ssl.c (ossl_start_ssl): ditto.

* test/openssl: check that OpenSSL.errors is empty every time after
  running a test case.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55051 b2dd03c8-39d4-4d8f-98ff-823fe69b080e