Fix underflow in BesselK

[NAThompson]

Closes #1229

[codecov]

Codecov Report

❌ Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.41%. Comparing base (3c1c431) to head (068497a).
⚠️ Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
.../boost/math/special_functions/detail/bessel_ik.hpp	90.90%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1448   +/-   ##
========================================
  Coverage    95.40%   95.41%           
========================================
  Files          831      831           
  Lines        69540    69572   +32     
========================================
+ Hits         66346    66379   +33     
+ Misses        3194     3193    -1     

Files with missing lines	Coverage Δ
.../boost/math/special_functions/detail/bessel_kn.hpp	100.00% <100.00%> (ø)
test/test_bessel_k.cpp	94.73% <100.00%> (+1.40%)	⬆️
.../boost/math/special_functions/detail/bessel_ik.hpp	98.96% <90.90%> (-1.04%)	⬇️

... and 3 files with indirect coverage changes

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[jzmaddock]

Thanks for this, does it fix my revised test case in #1229? Looks like it should?

[NAThompson]

@jzmaddock : Added your test case-cycling through CI now.

Take a look at your convenience.