fix(hangup): truthy check for deviceChangeListener before removing it (#3235)

It can be that deviceChangeListener is never defined because
the isDeviceList call never completes. On hangup, that would
cause an error to be thrown within lib-jitsi-meet because of
an attempt to remove an undefined event handler. That is
what happens on Safari right now.

conference.js

         APP.UI.removeLocalMedia();
 
         // Remove unnecessary event listeners from firing callbacks.
-        JitsiMeetJS.mediaDevices.removeEventListener(
-            JitsiMediaDevicesEvents.DEVICE_LIST_CHANGED,
-            this.deviceChangeListener);
+        if (this.deviceChangeListener) {
+            JitsiMeetJS.mediaDevices.removeEventListener(
+                JitsiMediaDevicesEvents.DEVICE_LIST_CHANGED,
+                this.deviceChangeListener);
+        }
 
         let requestFeedbackPromise;