Fix lower incomplete gamma functions with x = 0

[cohomology]

In this case, the errno error handling did not work correctly, as internal functions where accidently setting it, although no
overflow happens.

Fixes #1249.

[codecov]

Codecov Report

Attention: Patch coverage is 98.30508% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.84%. Comparing base (a5c0625) to head (6e5ba9a).

Files with missing lines	Patch %	Lines
include/boost/math/special_functions/gamma.hpp	90.90%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1250   +/-   ##
========================================
  Coverage    93.83%   93.84%           
========================================
  Files          657      658    +1     
  Lines        55244    55298   +54     
========================================
+ Hits         51840    51895   +55     
+ Misses        3404     3403    -1     

Files with missing lines	Coverage Δ
test/git_issue_1249.cpp	100.00% <100.00%> (ø)
include/boost/math/special_functions/gamma.hpp	92.18% <90.90%> (-0.08%)	⬇️

... and 1 file with indirect coverage changes

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