fix: download pdf twice may crash server

Signed-off-by: BoHong Li <raccoon@hackmd.io>

lib/note/noteActions.js

@@ -95,8 +95,11 @@ function actionPDF (req, res, note) {
     res.setHeader('Cache-Control', 'private')
     res.setHeader('Content-Type', 'application/pdf; charset=UTF-8')
     res.setHeader('X-Robots-Tag', 'noindex, nofollow') // prevent crawling
+    stream.on('end', () => {
+      stream.close()
+      fs.unlinkSync(pdfPath)
+    })
     stream.pipe(res)
-    fs.unlinkSync(pdfPath)
   })
 }