Prevent invalid number arithmetic from panicking

[terror]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.56098% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.56%. Comparing base (7aa9a79) to head (d8b0cfc).

Files with missing lines	Patch %	Lines
src/number.rs	95.34%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   78.29%   78.56%   +0.26%     
==========================================
  Files          29       30       +1     
  Lines        3456     3499      +43     
==========================================
+ Hits         2706     2749      +43     
  Misses        750      750              

Flag	Coverage Δ
unit	78.56% <97.56%> (+0.26%)	⬆️

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