http: make length/offset-related variables unsigned.

Fixes #5992, reported and found by Paul Cher <paulcher@icloud.com>.
static int parse_icy(HTTPContext *s, const char *tag, const char *p)
{
    int len = 4 + strlen(p) + strlen(tag);
    int is_first = !s->icy_metadata_headers;
    int ret;

    av_dict_set(&s->metadata, tag, p, 0);

    if (s->icy_metadata_headers)
        len += strlen(s->icy_metadata_headers);

    if ((ret = av_reallocp(&s->icy_metadata_headers, len)) < 0)
        return ret;

    if (is_first)
        *s->icy_metadata_headers = '\0';

    av_strlcatf(s->icy_metadata_headers, len, "%s: %s\n", tag, p);

    return 0;
}

static int parse_icy(HTTPContext *s, const char *tag, const char *p)
{
    int len = 4 + strlen(p) + strlen(tag);
    int is_first = !s->icy_metadata_headers;
    int ret;

    av_dict_set(&s->metadata, tag, p, 0);

    if (s->icy_metadata_headers)
        len += strlen(s->icy_metadata_headers);

    if ((ret = av_reallocp(&s->icy_metadata_headers, len)) < 0)
        return ret;

    if (is_first)
        *s->icy_metadata_headers = '\0';

    av_strlcatf(s->icy_metadata_headers, len, "%s: %s\n", tag, p);

    return 0;
}
