fix removeOnAudioProcess for Safari

When using "WebAudio" in Safari the "audioprocess" event doesn't get fired anymore once you pause/play the current song or load a new one into your wavesurfer object preventing the following piece of code to be executed:

this.backend.on('audioprocess', time => {
    this.drawer.progress(this.backend.getPlayedPercents());
    this.fireEvent('audioprocess', time);
});

This commit fixes the bug, so it is not needed anymore to use "MediaElement" on Safari as a workaround (which has its own drawbacks).

src/webaudio.js

@@ -253,7 +253,7 @@ export default class WebAudio extends util.Observer {
 
     /** @private */
     removeOnAudioProcess() {
-        this.scriptNode.onaudioprocess = null;
+        this.scriptNode.onaudioprocess = () => {};
     }
 
     /** @private */