fix(conference.js): crash on undefined

While on the prejoin screen, the local tracks are managed by the redux store and not conference.js, so localAudio is undefined.

conference.js

                     return this.useAudioStream(stream);
                 })
                 .then(() => {
-                    if (hasDefaultMicChanged) {
+                    if (this.localAudio && hasDefaultMicChanged) {
                         // workaround for the default device to be shown as selected in the
                         // settings even when the real device id was passed to gUM because of the
                         // above mentioned chrome bug.
                                     // Use the new stream or null if we failed to obtain it.
                                     return useStream(tracks.find(track => track.getType() === mediaType) || null)
                                         .then(() => {
-                                            if (hasDefaultMicChanged) {
+                                            if (this.localAudio && hasDefaultMicChanged) {
                                                 // workaround for the default device to be shown as selected in the
                                                 // settings even when the real device id was passed to gUM because of
                                                 // the above mentioned chrome bug.