http: make length/offset-related variables unsigned.

Fixes #5992, reported and found by Paul Cher <paulcher@icloud.com>.
static int parse_content_encoding(URLContext *h, const char *p)
{
    if (!av_strncasecmp(p, "gzip", 4) ||
        !av_strncasecmp(p, "deflate", 7)) {
#if CONFIG_ZLIB
        HTTPContext *s = h->priv_data;

        s->compressed = 1;
        inflateEnd(&s->inflate_stream);
        if (inflateInit2(&s->inflate_stream, 32 + 15) != Z_OK) {
            av_log(h, AV_LOG_WARNING, "Error during zlib initialisation: %s\n",
                   s->inflate_stream.msg);
            return AVERROR(ENOSYS);
        }
        if (zlibCompileFlags() & (1 << 17)) {
            av_log(h, AV_LOG_WARNING,
                   "Your zlib was compiled without gzip support.\n");
            return AVERROR(ENOSYS);
        }
#else
        av_log(h, AV_LOG_WARNING,
               "Compressed (%s) content, need zlib with gzip support\n", p);
        return AVERROR(ENOSYS);
#endif /* CONFIG_ZLIB */
    } else if (!av_strncasecmp(p, "identity", 8)) {
    } else {
        av_log(h, AV_LOG_WARNING, "Unknown content coding: %s\n", p);
    }
    return 0;
}

static int parse_content_encoding(URLContext *h, const char *p)
{
    if (!av_strncasecmp(p, "gzip", 4) ||
        !av_strncasecmp(p, "deflate", 7)) {
#if CONFIG_ZLIB
        HTTPContext *s = h->priv_data;

        s->compressed = 1;
        inflateEnd(&s->inflate_stream);
        if (inflateInit2(&s->inflate_stream, 32 + 15) != Z_OK) {
            av_log(h, AV_LOG_WARNING, "Error during zlib initialisation: %s\n",
                   s->inflate_stream.msg);
            return AVERROR(ENOSYS);
        }
        if (zlibCompileFlags() & (1 << 17)) {
            av_log(h, AV_LOG_WARNING,
                   "Your zlib was compiled without gzip support.\n");
            return AVERROR(ENOSYS);
        }
#else
        av_log(h, AV_LOG_WARNING,
               "Compressed (%s) content, need zlib with gzip support\n", p);
        return AVERROR(ENOSYS);
#endif /* CONFIG_ZLIB */
    } else if (!av_strncasecmp(p, "identity", 8)) {
    } else {
        av_log(h, AV_LOG_WARNING, "Unknown content coding: %s\n", p);
    }
    return 0;
}
