11: `CURLOPT_FORBID_REUSE` silently lost on multiplexed HTTP/2 connection when the forbidding transfer finishes first

When HTTP/2 transfers share a multiplexed connection, `CURLOPT_FORBID_REUSE` set by a transfer that completes before the other active stream(s) is silently discarded. The connection survives into the idle pool and can be reused by a subsequent transfer.

Curl_detach_connection(data);

if(CONN_INUSE(conn)) {
  /* Stop if still used. */
  return;
}