Fix test failures with gcc -O2

[biojppm]

Fixes #440

[codecov]

Codecov Report

Attention: Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.25%. Comparing base (e837309) to head (8c273eb).
Report is 26 commits behind head on master.

Files with missing lines	Patch %	Lines
src/c4/yml/parse.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #448   +/-   ##
=======================================
  Coverage   97.24%   97.25%           
=======================================
  Files          33       33           
  Lines       11259    11273   +14     
=======================================
+ Hits        10949    10963   +14     
  Misses        310      310           

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[biojppm]

For some reason the CI checks are not triggering. Running the gcc workflow manually confirms that the repro is working: see https://github.com/biojppm/rapidyaml/actions/runs/9717240536/job/26822517972 .

[biojppm]

This workflow shows the fix is working: https://github.com/biojppm/rapidyaml/actions/runs/9717245009/job/26822534347

[biojppm]

The failures are unrelated with this changeset, and will be dealt with separately. Merging now.