Fix typo.

Fix typo in ssl3_get_cert_verify: we can only skip certificate verify
message if certificate is absent.

NB: OpenSSL 0.9.8 is NOT vulnerable to CVE-2015-0205 as it doesn't
support DH certificates and this typo prohibits skipping of
certificate verify message for sign only certificates anyway.

Reviewed-by: Matt Caswell <matt@openssl.org>