db/simple: add missing "return"

SimpleDatabase::Visit() always throws exception when visiting a song.
Adding a song to the queue always results in a "No such directory"
error response, although the song is successfully added.  This
behaviour was introduced by commit
3ff728ab02.  This patch fixes this.

https://bugs.musicpd.org/view.php?id=4601

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -289,6 +289,8 @@ SimpleDatabase::Visit(const DatabaseSelection &selection,
 				const LightSong song2 = song->Export();
 				if (selection.Match(song2))
 					visit_song(song2);
+
+				return;
 			}
 		}
 	}