http: make length/offset-related variables unsigned.

Fixes #5992, reported and found by Paul Cher <paulcher@icloud.com>.
static int http_get_file_handle(URLContext *h)
{
    HTTPContext *s = h->priv_data;
    return ffurl_get_file_handle(s->hd);
}

static int http_get_file_handle(URLContext *h)
{
    HTTPContext *s = h->priv_data;
    return ffurl_get_file_handle(s->hd);
}
