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.

Co-authored-by: Saúl Ibarra Corretgé <saghul@jitsi.org>