free: prevent subtract with overflow on macOS

[cakebaker]

This PR should fix the "attempt to subtract with overflow" errors that make some tests fail on macOS. See, for example, https://github.com/uutils/procps/actions/runs/8505959792/job/23295337750?pr=37

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.42%. Comparing base (c5e7417) to head (b7d89f6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
- Coverage   45.53%   45.42%   -0.12%     
==========================================
  Files          15       15              
  Lines        1803     1803              
  Branches      240      240              
==========================================
- Hits          821      819       -2     
- Misses        853      854       +1     
- Partials      129      130       +1     

Flag	Coverage Δ
macos_latest	44.28% <100.00%> (ø)
ubuntu_latest	45.29% <ø> (-0.12%)	⬇️
windows_latest	0.13% <ø> (ø)

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