Fix for invalid subscriptions

[stasee]

When releasing a MediaObject, for example a PlayerEndpoint, there is no subscription on the server,
but the removeListener is called for 'EndOfStream'.
The subscriptions['EndOfStream'] does not exist and the token is invalid.
This fix prevents that the invalid 'token' is used, and that an Exception is raised.

[davidengelmaier]

This issue is based on a false assumption that the EventEmitter.listenerCount for the event will be 0 when the removeListener event is emitted, that is however not 100% sure as described in the documentation at https://www.tutorialspoint.com/nodejs/nodejs_event_emitter.htm:

This event is emitted any time someone removes a listener. When this event is triggered, the listener may not yet have been removed from the array of listeners for the event.

[jenkinskurento]

Hi there, thanks for your Pull Request!

A Kurento member needs to verify that this patch is reasonable to test. In case it is, they should write a comment with the phrase test this please. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by Kurento members will still work. Regular contributors can be whitelisted to skip this step.