e2ee: generate silence in case of audio decryption errors

This handles decryption errors for audio differently than the
current version. Instead of forwarding horrible noise to the decoder,
replace the bytes with magic opus bytes for silence:
  0xd8fffe

Those bytes were captured on a modified version of
  https://webrtc.github.io/samples/src/content/peerconnection/endtoend-encryption/
with more dumping and after disabling the track.

modules/e2ee/E2EEContext.js

@@ -314,6 +314,13 @@ export default class E2EEcontext {
                 return controller.enqueue(encodedFrame);
             }, e => {
                 logger.error(e);
+                if (encodedFrameType === undefined) { // audio, replace with silence.
+                    const newData = new ArrayBuffer(3);
+                    const newUint8 = new Uint8Array(newData);
+
+                    newUint8.set([ 0xd8, 0xff, 0xfe ]); // opus silence frame.
+                    encodedFrame.data = newData;
+                }
 
                 // Just feed the (potentially encrypted) frame in case of error.
                 // Worst case it is garbage.