There are two methods by which TLS clients signal the renegotiation
extension: either a special cipher suite value or a TLS extension.
It appears that I left debugging code in when I landed support for the
extension because there's a "+ 1" in the switch statement that shouldn't
be there.
The effect of this is very small, but it will break Firefox if
security.ssl.require_safe_negotiation is enabled in about:config.
(Although almost nobody does this.)
This change fixes the original bug and adds a test. Sadly the test is a
little complex because there's no OpenSSL s_client option that mirrors
that behaviour of require_safe_negotiation.