Fix integer overflow detected by fuzz test.

[dmuir]

No description provided.

[jkuhlmann]

Please excuse my ignorance, but what does this actually fix? Wouldn't it still overflow because type* and cgltf_size are of the same size?

[prideout]

I think this fixes the case where the macro argument (idx) has a type that's smaller than size_t. Seems like a good harmless fix.

[jkuhlmann]

Right, that way it makes sense. :-) Thanks!