event/SocketMonitor: don't filter out ERROR/HANGUP

By bit-wise ANDing the reported flags with GetScheduledFlags(),
ERROR/HANGUP always get cleared.  This means the MPD event loop could
never report those conditions.

src/event/SocketMonitor.cxx

@@ -30,7 +30,7 @@
 void
 SocketMonitor::Dispatch(unsigned flags) noexcept
 {
-	flags &= GetScheduledFlags();
+	flags &= GetScheduledFlags() | IMPLICIT_FLAGS;
 
 	if (flags != 0)
 		OnSocketReady(flags);