Use a null-terminated string for the file name

rust/coffi/src/main.rs

    unsafe {
        let mut img: png_image = std::mem::zeroed();
        print_img(&img);
        let res = png_image_begin_read_from_file(&mut img, "mei.png".as_ptr());
        let res = png_image_begin_read_from_file(&mut img, "mei.png\0".as_ptr());
        println!("read_from_file: {}", res);
        print_img(&img);
    }