`sqrt` of a tiny nonzero imaginary number returns exactly zero

[DRMacIver]

use num_complex::Complex;

let z = Complex::new(0.0_f64, 5e-324);
println!("{:?}", z.sqrt());

prints

Complex { re: 0.0, im: 0.0 }

z is nonzero (5e-324 is the smallest positive subnormal f64), but z.sqrt() is exactly 0. The true square root is about 1.6e-162 * (1 + i), a perfectly normal value; squaring the returned 0 cannot recover z.

Tested on num-complex 0.4.6.

BTW, this bug was found using hegel. Happy to contribute the tests if you're interested.