Main: use AtScopeExit() to call ZeroconfDeinit()

Make sure that ZeroconfDeinit() gets called even if startup fails with
an exception.  Fixes an assertion failure because an Avahi TimerEvent
is still active.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1192

src/Main.cxx

@@ -477,6 +477,7 @@ MainConfigured(const struct options &options, const ConfigData &raw_config)
 #endif
 
 	ZeroconfInit(raw_config, instance.event_loop);
+	AtScopeExit() { ZeroconfDeinit(); };
 
 #ifdef ENABLE_DATABASE
 	if (create_db) {
@@ -537,9 +538,6 @@ MainConfigured(const struct options &options, const ConfigData &raw_config)
 		instance.state_file->Write();
 
 	instance.BeginShutdownUpdate();
-
-	ZeroconfDeinit();
-
 	instance.BeginShutdownPartitions();
 }