BUG/MINOR: mux-quic: do not free conn if attached streams

Ensure via qcc_is_dead() that a connection is not released instance
until all of qcs streams are detached by the upper layer, even if an
error has been reported or the timeout has fired.

On the other side, as qc_detach() always check the connection status,
this should ensure that we do not keep a connection if not necessary.

Without this patch, a qcc instance may be freed with some of its qcs
streams not detached. This is an incorrect behavior and will lead to a
BUG_ON fault. Note however that no occurence of this bug has been
produced currently. This patch is mainly a safety against future
occurences.

This should be backported up to 2.6.

src/mux_quic.c

@@ -1001,7 +1001,12 @@ static void qcs_destroy(struct qcs *qcs)
 
 static inline int qcc_is_dead(const struct qcc *qcc)
 {
-	if ((qcc->conn->flags & CO_FL_ERROR) || !qcc->task)
+	/* Mux connection is considered dead if :
+	 * - all stream-desc are detached AND
+	 *   = connection is on error OR
+	 *   = mux timeout has already fired or is unset
+	 */
+	if (!qcc->nb_sc && ((qcc->conn->flags & CO_FL_ERROR) || !qcc->task))
 		return 1;
 
 	return 0;