client/Idle: consume only idle flags that were subscribed to

Since the very beginning when idle subscriptions where introduced
(commit 0bad84066b), waiting for a certain idle mask would clear
all other idle flags as well.  This would cause idle events to get
lost.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1998

src/client/Idle.cxx

@@ -26,9 +26,11 @@ void
 Client::IdleNotify() noexcept
 {
 	assert(idle_waiting);
-	assert(idle_flags != 0);
 
-	unsigned flags = std::exchange(idle_flags, 0) & idle_subscriptions;
+	const unsigned flags = idle_flags & idle_subscriptions;
+	idle_flags &= ~idle_subscriptions;
+	assert(flags != 0);
+
 	idle_waiting = false;
 
 	Response r(*this, 0);