Fix bias vs unbiased return in logb and ilogb

[mborland]

Closes: #1437

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.4%. Comparing base (012248c) to head (168117b).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1439     +/-   ##
=========================================
+ Coverage     98.3%   98.4%   +0.1%     
=========================================
  Files          295     296      +1     
  Lines        24642   24678     +36     
  Branches      2147    2148      +1     
=========================================
+ Hits         24223   24263     +40     
+ Misses         419     415      -4     

Files with missing lines	Coverage Δ
include/boost/decimal/detail/cmath/ilogb.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/logb.hpp	100.0% <100.0%> (+27.3%)	⬆️
test/github_issue_1437.cpp	100.0% <100.0%> (ø)
test/test_cmath.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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