fix(JingleSessionPC): avoid failures if replaceTrack runs late
fix(JingleSessionPC): avoid failures if replaceTrack runs late

Due to it being ran asyncronously, it's possible a replaceTrack operation is
scheduled while there is a valid PC, but by the time it runs was closed.

Accommodate to this situation by pretending everything is just peachy.

jiyeyuran/lib-jitsi-meet,jitsi/lib-jitsi-meet,adambowles/lib-jitsi-meet,adambowles/lib-jitsi-meet,jitsi/lib-jitsi-meet,damencho/lib-jitsi-meet,nevermind-corp/ljm