Main: save the state_file on shutdown

This got lost in commit 5d597a3646 (v0.21.19), but it was never
noticed because the state_file_interval was way too short due to
commit 3413d1bf23, fixed recently by commit 27cc7b352d

NEWS

@@ -2,6 +2,7 @@ ver 0.22.2 (not yet released)
 * database
   - simple: purge songs and virtual directories for unavailable plugins
     on update
+* state_file: save on shutdown
 
 ver 0.22.1 (2020/10/17)
 * decoder

src/Main.cxx

@@ -533,6 +533,9 @@ MainConfigured(const struct options &options, const ConfigData &raw_config)
 
 	/* cleanup */
 
+	if (instance.state_file)
+		instance.state_file->Write();
+
 	instance.BeginShutdownUpdate();
 
 	ZeroconfDeinit();