fix(hangup): ensure large video exists before getting displayed id (#2799)

On hangup while audio only, audio only is set to false on
conference leave to reset redux state on mobile. Large video will
update itself on conference leave, but large video has been cleaned
up by that time so trying to directly access the user ID on large
video will fail. Be defensive about this check, because its
callers are already defensive about its return value.

modules/UI/videolayout/VideoLayout.js

@@ -1092,7 +1092,7 @@ const VideoLayout = {
      * Currently used by tests (torture).
      */
     getLargeVideoID() {
-        return largeVideo.id;
+        return largeVideo && largeVideo.id;
     },
 
     /**