Fix case of finite x - inf not returning -inf

[mborland]

Closes: #1304

[codecov]

Codecov Report

❌ Patch coverage is 97.77778% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.9%. Comparing base (03b63a5) to head (8732359).
⚠️ Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/decimal32_t.hpp	75.0%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1305     +/-   ##
=========================================
- Coverage     98.9%   98.9%   -0.0%     
=========================================
  Files          272     273      +1     
  Lines        17792   17841     +49     
  Branches      1893    1904     +11     
=========================================
+ Hits         17596   17644     +48     
- Misses         196     197      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	96.8% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal64_t.hpp	95.3% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal_fast128_t.hpp	95.5% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal_fast32_t.hpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal_fast64_t.hpp	100.0% <100.0%> (ø)
test/github_issue_1304.cpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal32_t.hpp	96.4% <75.0%> (-0.2%)	⬇️

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