Fix access check in getSkeletonModelURLFromScript

[ksuprynowicz]

Big thanks to @AleziaKurdis for pinpointing this issue and providing test script. Earlier version was breaking flight avatar switcher app. The test script was reporting null on interface start with it:

//To reproduce, run this script in Edit>Running Scripts... (with this file on your machine)
//Do: File > Quit
//Launch Interface
//
//Expected: your avatar url
//Curruent: Null

var originalAvatarUrl = "";
originalAvatarUrl = MyAvatar.skeletonModelURL;

//The delay is because if I didn't put it, it was never displayed in the Log window (but it is maybe in the log file... i didn't check)
Script.setTimeout(function () {
    print("##########################################");
    print("FLYAVATAR originalAvatarUrl: " + originalAvatarUrl);
    print("##########################################");
}, 5000);