[Bug-gsl] cdf_poisson_P

[Georgios]

Dear GSL maintainer,

I think there is an issue with function *gsl_ran_poisson_pdf* (unsigned 
int k, double mu):

gsl_cdf_poisson_P(-1,5) returns 1.0000

while

gsl_cdf_poisson_P(1,5) returns 0.040428

The second number is fine, but the first one should be either equal to 
the second one, or if the sign makes a difference, it should be zero.

Best wishes,
Georgios