Y2106 bug in certificate expiration parsing.

In this code:

  const uint32_t expiration_date = rsa_ed_crosscert_get_expiration(cc);
  const uint64_t expiration_time = expiration_date * 3600;

Coverity caught this as CID 1415728.

No backport, since all current Tor releases will be obsolete by the time anyone hits this bug.