When listen only fallbacks from Kurento to FreeSWITCH, we must guarantee
the muted alert won't be created, speciallly because listen-only's fallback
uses a flow similar to microphone's. Client currently crashes when this
happens: this commit fixes this peoblem.
We now stop the MediaStream (cloned/dummy input stream) when this is not
needed anymore.
We don't create/clone a new MediaStream when it already exists.
Fixed eslint warning in muted-alert/component.jsx
