Fix related to Starling update

Hi,

Commit https://github.com/PrimaryFeather/Starling-Framework/commit/5250246d8e3ff1b6230ceb506c2f09ae6f056eb1 breaks this code because you might not have a frame for a sub texture anymore.
My patch fix the issue.
Merge pull request #204 from Fraggle/patch-2

Fix related to Starling update