Avoid division by 0 in beta_distribution operator()

[mborland]

Closes: #133

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.85%. Comparing base (59a5eb8) to head (c98dc05).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #134      +/-   ##
===========================================
+ Coverage    95.83%   95.85%   +0.01%     
===========================================
  Files          114      115       +1     
  Lines         8115     8126      +11     
===========================================
+ Hits          7777     7789      +12     
+ Misses         338      337       -1     

Files with missing lines	Coverage Δ
include/boost/random/beta_distribution.hpp	100.00% <100.00%> (ø)
test/github_issue_133.cpp	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[mborland]

The loop here keeps the shape of the distribution intact. I tried a simpler if (a == 0) return 0, but that skewed the distribution left.