Fix some gcc-13 warnings.

[jzmaddock]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.1%. Comparing base (5e1db68) to head (abc70a2).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #650   +/-   ##
=======================================
  Coverage     94.1%   94.1%           
=======================================
  Files          279     279           
  Lines        28903   28903           
=======================================
  Hits         27181   27181           
  Misses        1722    1722           

Files with missing lines	Coverage Δ
include/boost/multiprecision/cpp_bin_float.hpp	89.2% <ø> (ø)
...nclude/boost/multiprecision/detail/default_ops.hpp	90.9% <100.0%> (ø)

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