Sanity check frame object before adding to TextureCache

For texturepacker compatibility, make sure 'frame' property exists before attempting to use it.

I will concede that the patch name here is a little misleading. This may be an error specific to the project I am working on presently, and perhaps not attributed directly to the output of TexturePacker. But whatever the cause it seems ok to just be a little safer by sanity-checking the frame object before attempting to do something with it.

It seems that in some cases, the for in loop here encounters an `extend` property which of course doesn't contain a `frame` property, so the following error throws during the caching flow:

```Uncaught TypeError: Cannot read property 'x' of undefined```