Correct ellint_2 for N * PI / 2 phi argument.

[jzmaddock]

Fixes #1377.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.34%. Comparing base (7bb5ceb) to head (6f59f38).

Files with missing lines	Patch %	Lines
include/boost/math/special_functions/ellint_2.hpp	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1378      +/-   ##
===========================================
- Coverage    95.34%   95.34%   -0.01%     
===========================================
  Files          825      825              
  Lines        68160    68167       +7     
===========================================
+ Hits         64987    64992       +5     
- Misses        3173     3175       +2     

Files with missing lines	Coverage Δ
test/test_ellint_2.hpp	100.00% <100.00%> (ø)
include/boost/math/special_functions/ellint_2.hpp	99.21% <83.33%> (-0.79%)	⬇️

... and 1 file with indirect coverage changes

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