ClientList.cxx: copy client list before iterating

It seems that it is not safe to remove the item from std::list
while it's being iterated somewhere else.

This is a very simple quick fix to make things work
until some better solution is implemented.

src/ClientList.cxx

@@ -59,7 +59,8 @@ client_list_add(Client *client)
 void
 client_list_foreach(void (*callback)(Client *client, void *ctx), void *ctx)
 {
-	for (Client *client : clients)
+	auto clients_local = std::list<Client *>(clients);
+	for (Client *client : clients_local)
 		callback(client, ctx);
 }