Fix abs(nan) handling

[mborland]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.0%. Comparing base (51113cc) to head (a7704d2).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1259     +/-   ##
=========================================
+ Coverage     99.0%   99.0%   +0.1%     
=========================================
  Files          265     266      +1     
  Lines        17669   17682     +13     
  Branches      1980    1980             
=========================================
+ Hits         17475   17488     +13     
  Misses         194     194             

Files with missing lines	Coverage Δ
include/boost/decimal/detail/cmath/abs.hpp	100.0% <100.0%> (ø)

... and 7 files with indirect coverage changes

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