http: make length/offset-related variables unsigned.

Fixes #5992, reported and found by Paul Cher <paulcher@icloud.com>.
static int http_accept(URLContext *s, URLContext **c)
{
    int ret;
    HTTPContext *sc = s->priv_data;
    HTTPContext *cc;
    URLContext *sl = sc->hd;
    URLContext *cl = NULL;

    av_assert0(sc->listen);
    if ((ret = ffurl_alloc(c, s->filename, s->flags, &sl->interrupt_callback)) < 0)
        goto fail;
    cc = (*c)->priv_data;
    if ((ret = ffurl_accept(sl, &cl)) < 0)
        goto fail;
    cc->hd = cl;
    cc->is_multi_client = 1;
fail:
    return ret;
}

static int http_accept(URLContext *s, URLContext **c)
{
    int ret;
    HTTPContext *sc = s->priv_data;
    HTTPContext *cc;
    URLContext *sl = sc->hd;
    URLContext *cl = NULL;

    av_assert0(sc->listen);
    if ((ret = ffurl_alloc(c, s->filename, s->flags, &sl->interrupt_callback)) < 0)
        goto fail;
    cc = (*c)->priv_data;
    if ((ret = ffurl_accept(sl, &cl)) < 0)
        goto fail;
    cc->hd = cl;
    cc->is_multi_client = 1;
fail:
    return ret;
}
