Fix handling of inf times negative values

[mborland]

Closes: #1302

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.9%. Comparing base (16c3d27) to head (958ff9a).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1303     +/-   ##
=========================================
+ Coverage     98.9%   98.9%   +0.1%     
=========================================
  Files          271     272      +1     
  Lines        17683   17792    +109     
  Branches      1886    1893      +7     
=========================================
+ Hits         17487   17595    +108     
- Misses         196     197      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	96.8% <100.0%> (+0.2%)	⬆️
include/boost/decimal/decimal32_t.hpp	96.7% <100.0%> (+0.2%)	⬆️
include/boost/decimal/decimal64_t.hpp	95.3% <100.0%> (+0.2%)	⬆️
include/boost/decimal/decimal_fast128_t.hpp	95.4% <100.0%> (+0.3%)	⬆️
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_1302.cpp	100.0% <100.0%> (ø)
test/test_to_chars.cpp	100.0% <ø> (ø)

... 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 16c3d27...958ff9a. Read the comment docs.

🚀 New features to boost your workflow:

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