Prevent spurious numeric underflow in ibeta_series.

[jzmaddock]

Fixes #1247.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.84%. Comparing base (1290be6) to head (f0ef1fd).
Report is 8 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1260   +/-   ##
========================================
  Coverage    93.84%   93.84%           
========================================
  Files          658      659    +1     
  Lines        55299    55311   +12     
========================================
+ Hits         51895    51907   +12     
  Misses        3404     3404           

Files with missing lines	Coverage Δ
include/boost/math/special_functions/beta.hpp	100.00% <100.00%> (ø)
test/git_issue_1247.cpp	100.00% <100.00%> (ø)

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