Fix: handle zip-download for not existing bundle

lib/endpoints.js

@@ -86,6 +86,8 @@ app.get('/files/:fid', async(req, res, next) => {
     const format = req.params.fid.endsWith('.zip') ? 'zip' : 'tar.gz';
     const bucket = db.get(sid);
 
+    if(!bucket) return res.status(404).send(errorPage.replace('%%ERROR%%', 'Download bucket not found.'));
+
     if(req.params.fid !== sid + '++' + MD5(bucket.map(f => f.key).join()).toString() + '.' + format) {
       res.status(404).send(errorPage.replace('%%ERROR%%', 'Invalid link'));
       return;