cert: fix clippy get_first finding

[cpu]

Scheduled CI tasks started failing yesterday with the update to the just-released Rust 1.75 from a clippy finding:

error: accessing first element with `crl_distribution_points.get(0)`
   --> src/cert.rs:620:13
    |
620 | /             crl_distribution_points
621 | |                 .get(0)
    | |_______________________^ help: try: `crl_distribution_points.first()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first
    = note: `-D clippy::get-first` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::get_first)]`

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b528297) 97.16% compared to head (e98d948) 97.16%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #220   +/-   ##
=======================================
  Coverage   97.16%   97.16%           
=======================================
  Files          19       19           
  Lines        4303     4303           
=======================================
  Hits         4181     4181           
  Misses        122      122           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.