Use SignedAmount::unsigned_abs to avoid overflow

[jkczyz]

In debug mode, using SignedAmount::abs can lead to an integer overflow when used with SignedAmount::MIN. Use SignedAmount::unsigned_abs to avoid this.

[ldk-reviews-bot]

👋 Thanks for assigning @TheBlueMatt as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[wpaulino]

Worth adding a test?

[TheBlueMatt]

Meh, not sure its worth bothering to test a bug that isn't even a bug in prod. The appropriate test would be a CI check that bans the abs method in our code, anyway.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.01%. Comparing base (817ab5e) to head (2d948fd).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4383      +/-   ##
==========================================
- Coverage   86.01%   86.01%   -0.01%     
==========================================
  Files         156      156              
  Lines      102857   102857              
  Branches   102857   102857              
==========================================
- Hits        88476    88473       -3     
- Misses      11871    11875       +4     
+ Partials     2510     2509       -1     

Flag	Coverage Δ
tests	86.01% <100.00%> (-0.01%)	⬇️

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