test_x509req.rb: fix for RHEL7

* test/openssl/test_x509req.rb (test_sign_and_verify_rsa_md5): fix
  exception to be rescued.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46904 b2dd03c8-39d4-4d8f-98ff-823fe69b080e