OpenSSL: don't allow the server certificate to change during renegotiation.

      This mirrors r229611, but for OpenSSL.

BUG=306959

Review URL: https://codereview.chromium.org/177143004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254022 0039d316-1c4b-4281-b951-d872f2087c98