zstd cannot match when accept-encoding is gzip,zstd,br

[drawing]

cannot match zstd when accept-encoding is gzip,zstd,br.

static ngx_int_t
ngx_http_zstd_accept_encoding(ngx_str_t *ae)
{
    u_char  *p;

    p = ngx_strcasestrn(ae->data, "zstd", sizeof("zstd") - 1);
    if (p == NULL) {
        return NGX_DECLINED;
    }

    if (p == ae->data || (*(p - 1) == ',' || *(p - 1) == ' ')) {

        p += sizeof("zstd") - 1;

        if (p == ae->data + ae->len || *p == ',' || *p == ' ' || *p == ';') {
            return NGX_OK;
        }
    }

    return NGX_DECLINED;
}

u_char * ngx_strcasestrn(u_char *s1, char *s2, size_t n)

The argument n of ngx_strcasestrn must be length of the second substring - 1. But the length 4 is passed in the code.