Fix an integer overflow in the CRL signature parser.

* src/crl.c (parse_signature): N+N2 now checked for overflow.
* src/ocsp.c (parse_response_extensions): Do not accept too large
values.
(parse_single_extensions): Ditto.

The second patch is an extra safegourd not related to the reported
bug.

• GnuPG-bug-id: T6284
• Reported-by: Joseph Surin, elttam