Add some sanity checks when checking CRL scores

Note: this was accidentally omitted from OpenSSL 1.0.2 branch.
Without this fix any attempt to use CRLs will crash.

CVE-2016-7052

Thanks to Bruce Stephens and Thomas Jakobi for reporting this issue.

Reviewed-by: Stephen Henson <steve@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>