Fix handling of denorm next after

[mborland]

Closes: #1105

[codecov]

Codecov Report

❌ Patch coverage is 97.95918% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.4%. Comparing base (7cfb824) to head (8d2bf74).
⚠️ Report is 11 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/decimal64_t.hpp	88.9%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1146     +/-   ##
=========================================
- Coverage     99.4%   99.4%   -0.0%     
=========================================
  Files          252     252             
  Lines        17164   17208     +44     
  Branches      1866    1870      +4     
=========================================
+ Hits         17048   17090     +42     
- Misses         116     118      +2     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	98.7% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal32_t.hpp	96.8% <100.0%> (+0.1%)	⬆️
include/boost/decimal/detail/cmath/next.hpp	100.0% <100.0%> (ø)
test/github_issue_1105.cpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal64_t.hpp	96.6% <88.9%> (-0.1%)	⬇️

... and 1 file with indirect coverage changes

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