rend_parse_client_keys() prints stack in logs if base64_decode fails

int
rend_parse_client_keys(strmap_t *parsed_clients, const char *ckstr)
{
...
    char descriptor_cookie_base64[REND_DESC_COOKIE_LEN_BASE64+2+1];
...
    /* The size of descriptor_cookie_tmp needs to be REND_DESC_COOKIE_LEN+2,
     * because a base64 encoding of length 24 does not fit into 16 bytes in all
     * cases. */
    if ((base64_decode(descriptor_cookie_tmp, REND_DESC_COOKIE_LEN+2,
                       tok->args[0], REND_DESC_COOKIE_LEN_BASE64+2+1)
           != REND_DESC_COOKIE_LEN)) {
      log_warn(LD_REND, "Descriptor cookie contains illegal characters: "
                        "%s", descriptor_cookie_base64);
      goto err;
    }
...

descriptor_cookie_base64 was never initialized, so it upon base64_decode() failure, it prints stack garbage to the logs.

Not an important bug, but a bug alright, so I'm putting it here to not forget it.