Fix creating offers again in Firefox with disabled media and simulcast

When a media track is disabled the track is nullified in the sender, so
this needs to be taken into account when creating a new offer on an
existing connection with disabled media (for example, to add a video
track to a connection with an active but disabled audio track).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

Author: danxuliu

src/utils/webrtc/simplewebrtc/peer.js

@@ -356,7 +356,7 @@ Peer.prototype.offer = function(options) {
 	if (sendVideo && this.enableSimulcast && adapter.browserDetails.browser === 'firefox') {
 		console.debug('Enabling Simulcasting for Firefox (RID)')
 		const sender = this.pc.getSenders().find(function(s) {
-			return s.track.kind === 'video'
+			return s.track && s.track.kind === 'video'
 		})
 		if (sender) {
 			let parameters = sender.getParameters()