Fix log2 computation

[mborland]

Closes: #275

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.89%. Comparing base (e39c280) to head (5e2133a).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #276   +/-   ##
========================================
  Coverage    94.89%   94.89%           
========================================
  Files           71       71           
  Lines         9123     9123           
========================================
  Hits          8657     8657           
  Misses         466      466           

Files with missing lines	Coverage Δ
.../boost/charconv/detail/from_chars_integer_impl.hpp	100.00% <100.00%> (ø)

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