Fix construction of subnormal values

[mborland]

Closes: #1306

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.9%. Comparing base (c46e972) to head (5d83a8a).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1307     +/-   ##
=========================================
+ Coverage     98.9%   98.9%   +0.1%     
=========================================
  Files          273     274      +1     
  Lines        17841   17867     +26     
  Branches      1904    1908      +4     
=========================================
+ Hits         17643   17669     +26     
  Misses         198     198             

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	96.9% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal32_t.hpp	96.5% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal64_t.hpp	95.4% <100.0%> (+0.1%)	⬆️
include/boost/decimal/detail/cmath/next.hpp	95.2% <100.0%> (-2.3%)	⬇️
test/github_issue_1105.cpp	100.0% <100.0%> (ø)
test/github_issue_1306.cpp	100.0% <100.0%> (ø)

... and 3 files with indirect coverage changes

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