Thread panics on EXR containing NAN

[gyk]

Some EXR files might contain NaNs, e.g., AllHalfValues.exr. Calling to_rgba8 on such images causes a panic:

thread 'main' panicked at image-0.25.1\src\color.rs:435:30:
called `Option::unwrap()` on a `None` value

[fintelia]

Shorter repro:

let img = Rgba32FImage::from_raw(1, 1, vec![1.0 / 0.]).expect("create nan image");
DynamicImage::ImageRgba32F(img).to_rgba8();