PKINIT null pointer deref [CVE-2013-1415]

Don't dereference a null pointer when cleaning up.

The KDC plugin for PKINIT can dereference a null pointer when a
malformed packet causes processing to terminate early, leading to
a crash of the KDC process.  An attacker would need to have a valid
PKINIT certificate or have observed a successful PKINIT authentication,
or an unauthenticated attacker could execute the attack if anonymous
PKINIT is enabled.

CVSSv2 vector: AV:N/AC:M/Au:N/C:N/I:N/A:C/E:P/RL:O/RC:C

This is a minimal commit for pullup; style fixes in a followup.
[kaduk@mit.edu: reformat and edit commit message]

(cherry picked from commit c773d3c775e9b2d88bcdff5f8a8ba88d7ec4e8ed)

ticket: 7570
version_fixed: 1.11.1
status: resolved
crypto_retrieve_cert_sans(krb5_context context,
                          pkinit_plg_crypto_context plgctx,
                          pkinit_req_crypto_context reqctx,
                          pkinit_identity_crypto_context idctx,
                          krb5_principal **princs_ret,
                          krb5_principal **upn_ret,
                          unsigned char ***dns_ret)
{
    krb5_error_code retval = EINVAL;

    if (reqctx->received_cert == NULL) {
        pkiDebug("%s: No certificate!\n", __FUNCTION__);
        return retval;
    }

    return crypto_retrieve_X509_sans(context, plgctx, reqctx,
                                     reqctx->received_cert, princs_ret,
                                     upn_ret, dns_ret);
}

crypto_retrieve_cert_sans(krb5_context context,
                          pkinit_plg_crypto_context plgctx,
                          pkinit_req_crypto_context reqctx,
                          pkinit_identity_crypto_context idctx,
                          krb5_principal **princs_ret,
                          krb5_principal **upn_ret,
                          unsigned char ***dns_ret)
{
    krb5_error_code retval = EINVAL;

    if (reqctx->received_cert == NULL) {
        pkiDebug("%s: No certificate!\n", __FUNCTION__);
        return retval;
    }

    return crypto_retrieve_X509_sans(context, plgctx, reqctx,
                                     reqctx->received_cert, princs_ret,
                                     upn_ret, dns_ret);
}
