squid: weak certificate validation

The flaw allows remote servers to bypass client certificate validation.
Some attackers may also be able to use valid certificates for one domain
signed by a global Certificate Authority to abuse an unrelated domain.
However, the bug is exploitable only if you have configured Squid to
perform SSL Bumping with the "client-first" or "bump" mode of operation.
Sites that do not use SSL-Bump are not vulnerable.