Initialize the `.version` field correctly

It must be set to PNG_IMAGE_VERSION, which seems to be one, currently.

rust/coffi/src/main.rs

@@ -42,6 +42,7 @@ fn main() {
    unsafe {
        let mut img: png_image = std::mem::zeroed();
        img.version = 1;
        print_img(&img);
        let res = png_image_begin_read_from_file(&mut img, "mei.png\0".as_ptr());
        println!("read_from_file: {}", res);