fix(p2p) fix error if p2p session is stopped while accepting it

The _MEDIA_SESSION_STARTED would be emitted with `null` as the session, instead
of the original one.

JitsiConference.js

 
             this.eventEmitter.emit(
                 JitsiConferenceEvents._MEDIA_SESSION_STARTED,
-                this.p2pJingleSession);
+                jingleSession);
         },
         error => {
             logger.error(