Avoid panic in an internal base_cache::Clocks::to_std_instant method

[tatsuya6502]

Fixes #472.

This PR should avoid occasional panic in an internal base_cache::Clocks::to_std_instant method by using zero Duration if checked_duration_since method fails.

I could not figure out why the specific call of checked_duration_since method can fail. I might have overlooked something in moka's code, or maybe quanta::Instant can return not a monotonically increasing time on some CPUs?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.30%. Comparing base (e740de9) to head (93be05b).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #477   +/-   ##
=======================================
  Coverage   94.29%   94.30%           
=======================================
  Files          43       43           
  Lines       20449    20449           
=======================================
+ Hits        19283    19285    +2     
+ Misses       1166     1164    -2     

[tatsuya6502]

Merging.