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

NEWS

@@ -14,6 +14,7 @@ ver 0.24 (not yet released)
   - new "search"/"find" filter "added-since"
   - allow range in listplaylist and listplaylistinfo
   - "sticker find" supports sort and window parameter and new sticker compare operators "eq", "lt" and "gt"
+  - consume only idle flags that were subscribed to
 * database
   - attribute "added" shows when each song was added to the database
   - fix integer overflows with 64-bit inode numbers

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);