db/DatabasePlaylist: increment only one variable

Fixes "searchaddpl" bug emitting bogus error "Bad position".

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1338

src/db/DatabasePlaylist.cxx

@@ -53,10 +53,9 @@ SearchInsertIntoPlaylist(const Database &db, const Storage *storage,
 
 	unsigned n = 0;
 
-	db.Visit(selection, [&playlist, &position, &n, storage](const auto &song){
+	db.Visit(selection, [&playlist, position, &n, storage](const auto &song){
 		playlist.Insert(position + n,
 				DatabaseDetachSong(storage, song));
-		++position;
 		++n;
 	});