String#succ because it can make modified (wrong) auth_tag longer
  than 16 bytes.  The longer auth_tag makes that
  EVP_CIPHER_CTX_ctrl (and internally aes_gcm_ctrl) fail.
  [ruby-core:55143] [Bug #8439] reported by Vit Ondruch.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43676 b2dd03c8-39d4-4d8f-98ff-823fe69b080e