Fix non-finite value handling in operator%

[mborland]

Closes: #1319

[codecov]

Codecov Report

❌ Patch coverage is 78.44828% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.8%. Comparing base (bd99e1d) to head (41a22b6).
⚠️ Report is 10 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/decimal_fast128_t.hpp	62.5%	6 Missing ⚠️
include/boost/decimal/decimal128_t.hpp	75.0%	4 Missing ⚠️
include/boost/decimal/decimal32_t.hpp	75.0%	4 Missing ⚠️
include/boost/decimal/decimal_fast32_t.hpp	75.0%	4 Missing ⚠️
include/boost/decimal/decimal_fast64_t.hpp	75.0%	4 Missing ⚠️
include/boost/decimal/decimal64_t.hpp	81.3%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1320     +/-   ##
=========================================
- Coverage     99.0%   98.8%   -0.1%     
=========================================
  Files          276     277      +1     
  Lines        17902   18008    +106     
  Branches      1909    1915      +6     
=========================================
+ Hits         17706   17785     +79     
- Misses         196     223     +27     

Files with missing lines	Coverage Δ
test/github_issue_1319.cpp	100.0% <100.0%> (ø)
test/random_decimal64_math.cpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal64_t.hpp	94.8% <81.3%> (-0.5%)	⬇️
include/boost/decimal/decimal128_t.hpp	97.2% <75.0%> (-0.9%)	⬇️
include/boost/decimal/decimal32_t.hpp	95.7% <75.0%> (-0.8%)	⬇️
include/boost/decimal/decimal_fast32_t.hpp	98.8% <75.0%> (-1.2%)	⬇️
include/boost/decimal/decimal_fast64_t.hpp	98.7% <75.0%> (-1.3%)	⬇️
include/boost/decimal/decimal_fast128_t.hpp	95.3% <62.5%> (-1.5%)	⬇️

... and 2 files with indirect coverage changes

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