gsl_ran_negative_binomial_pdf

[Redekopp, Eric]

gsl_ran_negative_binomial_pdf gives -NaN if p == 1.0, despite p=1 being a valid 
negative binomial distribution. I believe this is because of the call to 
log1p(-p) which evaluates to log(0) and gives NaN. There appear to be guards 
against this in the positive version (gsl_ran_binomial_pdf) and it works as 
expected, however these edge cases aren't covered in the negative version. I 
think that the negative version should have similar guards to return 
appropriate values for edge cases like p==1.

Thanks for your consideration,
Eric Redekopp
University of Saskatchewan