Minimal wkhtmltopdf snippet that works in debug but segfaults in --release

wkhtmltopdf-release-segfault.rs

extern crate libwkhtmltox_sys as libwkhtmltox; // https://github.com/anowell/libwkhtmltox-sys

use libwkhtmltox::*;
use std::ffi::{CString, CStr};
use std::os::raw::{c_char, c_int, c_uchar};

unsafe extern fn finished_callback(converter: *mut wkhtmltopdf_converter, val: c_int) {
    println!("finished_callback: {}", val);

    // Read the PDF output
    let buf_ptr: *mut *const c_uchar = std::mem::uninitialized();
    println!("calling wkhtmltopdf_get_output");
    let bytes = wkhtmltopdf_get_output(converter, buf_ptr) as usize;
    println!("output received");
    let buf_slice = std::slice::from_raw_parts(*buf_ptr as *const c_uchar, bytes);
    println!("buf_slice.len={}", buf_slice.len());
}

unsafe extern fn error_callback(_converter: *mut wkhtmltopdf_converter, ptr: *const c_char) {
    let cstr = CStr::from_ptr(ptr);
    let msg = cstr.to_string_lossy().into_owned();
    println!("error_callback: {}", msg);
}

fn main() {
    unsafe {
        if wkhtmltopdf_init(0) != 1 {
            panic!("init failed");
        }

        let global_settings = wkhtmltopdf_create_global_settings();
        let object_settings = wkhtmltopdf_create_object_settings();
        let converter = wkhtmltopdf_create_converter(global_settings);

        // Setup callbacks
        wkhtmltopdf_set_finished_callback(converter, Some(finished_callback));
        wkhtmltopdf_set_error_callback(converter, Some(error_callback));

        // Perform the conversion
        let c_html = CString::new(r##"<b>foo</b>bar"##).expect("null byte found");
        wkhtmltopdf_add_object(converter, object_settings, c_html.as_ptr());
        if wkhtmltopdf_convert(converter) != 1 {
            panic!("wktmltopdf_convert failed");
        }

        // Wait a bitfor the callbacks a chance to run
        std::thread::sleep(std::time::Duration::from_secs(4));
    }
}