pFq: prevent spurious overflow.

[jzmaddock]

Make sure we throw the correct exceptions when the result is infinite. Fixes #1404.

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.40%. Comparing base (9a964b4) to head (ad99070).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...tions/detail/hypergeometric_pFq_checked_series.hpp	87.50%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1406   +/-   ##
========================================
  Coverage    95.39%   95.40%           
========================================
  Files          826      826           
  Lines        68901    68919   +18     
========================================
+ Hits         65729    65750   +21     
+ Misses        3172     3169    -3     

Files with missing lines	Coverage Δ
...oost/math/special_functions/hypergeometric_pFq.hpp	100.00% <100.00%> (ø)
test/test_pFq.hpp	89.21% <100.00%> (+1.43%)	⬆️
...tions/detail/hypergeometric_pFq_checked_series.hpp	90.09% <87.50%> (+0.89%)	⬆️

---

Continue to review full report in Codecov by Harness.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9a964b4...ad99070. Read the comment docs.

🚀 New features to boost your workflow:

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