powf returns NaN for small bases

[mb64]

When trying to port palette to support #[no_std], its extensive tests found some scenarios where mish functions returned NaN.

powf returns NaN fairly consistently, no matter the exponent, for bases smaller than 0.08518821616297091.

Ex:

extern crate mish;
for &power in [0.00001, 0.3, 0.9999, 1.0, 2.0, 10000.0].iter() {
    assert!(mish::pow::<f64>(0.085, power).is_nan());
}