Use `zeroed` instead of `unitialized`.

It's at least clear if it worked, then.

rust/coffi/src/main.rs

    let mut img: png_image;
    unsafe {
        img = std::mem::uninitialized();
        img = std::mem::zeroed();
        let res = png_image_begin_read_from_file(&mut img, "mei.png".as_ptr());
        println!("read_from_file: {}", res);
        println!("{}x{}", img.width, img.height);