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

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