Poisson distribution falls into an infinite loop for parameter λ=∞

[Thopic]

When the parameter of the Poisson distribution is set to f64::INFINITY the sample function doesn't panic but instead loop and never stops. For example:

use rand_distr::{Poisson, Distribution};
fn main() {
    let lambda = f64::INFINITY;
    let poi = Poisson::new(lambda).unwrap();
    println!("{:?}", poi.sample(&mut rand::thread_rng()));
}

[dhardy]

Sounds like Poisson::new should return an error (and the Error type wants an extra state for this).

A PR would be welcome (please include a note in the CHANGELOG).