GH-3406 testing option of crash due to subtraction overflow

[DmitryBespalov]

Handles #3406

Changes proposed in this pull request:

• Avoid overflow on subtraction

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 8.25%. Comparing base (fb255a5) to head (76753a7).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...Management/KeystoneOwnerKey/KeystoneSignFlow.swift	0.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #3421      +/-   ##
========================================
- Coverage   8.25%   8.25%   -0.01%     
========================================
  Files        788     788              
  Lines      44211   44214       +3     
========================================
  Hits        3648    3648              
- Misses     40563   40566       +3     

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