Fix non-finite handling in fmax and fmin

[mborland]

Closes: #1299

[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.9%. Comparing base (322ffce) to head (ead1547).
⚠️ Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
test/github_issue_1299.cpp	97.0%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1300     +/-   ##
=========================================
- Coverage     98.9%   98.9%   -0.0%     
=========================================
  Files          270     271      +1     
  Lines        17607   17683     +76     
  Branches      1883    1886      +3     
=========================================
+ Hits         17411   17486     +75     
- Misses         196     197      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/detail/cmath/fmax.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/fmin.hpp	100.0% <100.0%> (+14.3%)	⬆️
include/boost/decimal/detail/cmath/nan.hpp	92.9% <100.0%> (+0.3%)	⬆️
test/github_issue_1299.cpp	97.0% <97.0%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 322ffce...ead1547. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.