Fix division near 0

[mborland]

The values were often and incorrectly being set to infinity instead of zero. Also fixes the include guard for decimal128_fast since the IDE warned me about it.

Closes: #808

[codecov]

Codecov Report

Attention: Patch coverage is 93.47826% with 3 lines in your changes missing coverage. Please review.

Project coverage is 99.0%. Comparing base (fd91d52) to head (033707d).
Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/decimal128.hpp	75.0%	1 Missing ⚠️
include/boost/decimal/decimal32.hpp	75.0%	1 Missing ⚠️
include/boost/decimal/decimal64.hpp	75.0%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #877     +/-   ##
=========================================
+ Coverage     98.9%   99.0%   +0.1%     
=========================================
  Files          234     235      +1     
  Lines        16805   16840     +35     
  Branches      1797    1800      +3     
=========================================
+ Hits         16620   16659     +39     
+ Misses         185     181      -4     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_fast.hpp	98.3% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal32_fast.hpp	96.0% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal64_fast.hpp	98.0% <100.0%> (+0.1%)	⬆️
test/github_issue_798.cpp	100.0% <100.0%> (ø)
test/github_issue_808.cpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal128.hpp	97.9% <75.0%> (+0.1%)	⬆️
include/boost/decimal/decimal32.hpp	98.6% <75.0%> (+0.1%)	⬆️
include/boost/decimal/decimal64.hpp	99.1% <75.0%> (+0.1%)	⬆️

... and 1 file with indirect coverage changes

---

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 fd91d52...033707d. Read the comment docs.