Fix an error when video playback buffer is empty (#5300)

app/javascript/mastodon/features/video/index.js

@@ -210,8 +210,10 @@ export default class Video extends React.PureComponent {
   }
 
   handleProgress = () => {
+    if (this.video.buffered.length > 0) {
       this.setState({ buffer: this.video.buffered.end(0) / this.video.duration * 100 });
     }
+  }
 
   handleOpenVideo = () => {
     this.video.pause();