Fix closing of AudioContext that is already closed

When you were retrying after being done, the 'retry' handler was
trying to close the AudioContext that had already been closed by
the 'done' handler thus throwing an exception. Fixed by checking
the AudioContext's state for being closed already before trying
to close it.

src/components/Recorder.js

@@ -248,7 +248,9 @@ export default class Recorder extends H5P.EventDispatcher {
     this.stream.getAudioTracks().forEach(track => track.stop());
     this.sourceNode.disconnect();
     this.scriptProcessorNode.disconnect();
-    this.audioContext.close();
+    if (this.audioContext.state !== 'closed') {
+      this.audioContext.close();
+    }
 
     delete this.userMedia;
   }