Remove use of ops::Mul and ops::Neg since they can fail

[jonasbb]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 66.37%. Comparing base (4c273b2) to head (90d7312).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
serde_with/src/utils/duration.rs	84.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #811      +/-   ##
==========================================
- Coverage   66.40%   66.37%   -0.03%     
==========================================
  Files          40       40              
  Lines        2625     2635      +10     
==========================================
+ Hits         1743     1749       +6     
- Misses        882      886       +4     

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