Fix non-finite division handling

[mborland]

Closes: #1312

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.9%. Comparing base (9a4989b) to head (fc8fce1).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1313     +/-   ##
=========================================
+ Coverage     98.9%   98.9%   +0.1%     
=========================================
  Files          274     275      +1     
  Lines        17869   17886     +17     
  Branches      1908    1908             
=========================================
+ Hits         17671   17688     +17     
  Misses         198     198             

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	96.9% <100.0%> (ø)
include/boost/decimal/decimal32_t.hpp	96.5% <100.0%> (ø)
include/boost/decimal/decimal64_t.hpp	95.4% <100.0%> (ø)
include/boost/decimal/decimal_fast128_t.hpp	95.5% <100.0%> (ø)
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_1312.cpp	100.0% <100.0%> (ø)
test/random_decimal64_math.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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