According to http://xmpp.org/rfcs/rfc3920.html#sasl (section 6.4), the correct server response would be "<not-authorized/>". "<forbidden/>" is no valid response. As it is important that anyone can create a full-featured XMPP client by only using the official open protocols, it is *forbidden* to apply this patch to Coccinella.
Of course, Coccinella should not segfault (it doesn't do this on Linux btw). So, instead, I would suggest to rewrite the code to use a switch command. If the value is not found, this following string should be showed:
"Invalid error. Bug in server software: error <%s/> not defined in section 6.4 SASL Errors of RFC 3920. Please consider reporting this bug to your server administrator."