fix check playlist existence before deleting it

store/files.go

@@ -100,6 +100,7 @@ func (st *Store) GetFileUsage(group string, id uint64) (playlists Playlists, err
 
 func (st *Store) DeleteFile(group string, id uint64) error {
 	tx := st.db.Begin()
+	// make sure the file exists and actually belongs to <group>
 	if err := tx.Where("group_name = ?", group).First(&File{}, id).Error; err != nil {
 		tx.Rollback()
 		return err

store/playlists.go

@@ -109,7 +109,8 @@ func (st *Store) UpdatePlaylist(group string, id uint64, playlist Playlist) (*Pl
 
 func (st *Store) DeletePlaylist(group string, id uint64) (err error) {
 	tx := st.db.Begin()
-	if err = tx.First(&Playlist{}, id).Error; err != nil {
+	// make sure the playlist exists and actually belongs to <group>
+	if err = tx.Where("group_name = ?", group).First(&Playlist{}, id).Error; err != nil {
 		tx.Rollback()
 		return
 	}