Fix erroneous cast to unrepresentable values

[mborland]

Closes: #1361

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.6%. Comparing base (a07d93a) to head (c44572c).
⚠️ Report is 27 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1362     +/-   ##
=========================================
- Coverage     98.6%   98.6%   -0.0%     
=========================================
  Files          283     284      +1     
  Lines        22625   22656     +31     
  Branches      1972    1973      +1     
=========================================
+ Hits         22292   22322     +30     
- Misses         333     334      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/detail/to_integral.hpp	92.2% <100.0%> (-3.3%)	⬇️
test/github_issue_1361.cpp	100.0% <100.0%> (ø)

... and 4 files 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 a07d93a...c44572c. Read the comment docs.

🚀 New features to boost your workflow:

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