Fix hang in 1F1

[NAThompson]

No description provided.

[NAThompson]

Closes #1288

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.41%. Comparing base (1187ea2) to head (729986d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1446   +/-   ##
========================================
  Coverage    95.41%   95.41%           
========================================
  Files          831      831           
  Lines        69521    69522    +1     
========================================
+ Hits         66330    66331    +1     
  Misses        3191     3191           

Files with missing lines	Coverage Δ
...oost/math/special_functions/hypergeometric_1F1.hpp	100.00% <100.00%> (ø)
test/test_1F1.cpp	96.55% <100.00%> (+0.12%)	⬆️

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[NAThompson]

@jzmaddock : take a look at your convenience.

[jzmaddock]

Drone failures are all spurious, not sure what's going on there, but merging...