Fix rounding errors in calculation of minimum change size
Fix rounding errors in calculation of minimum change size

Summary:
Backport of Core PR11145

The logging part of it is uneeded for us.

Test Plan:
  make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3291

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