Blender Git Loki

Attempt to fix T45718, crash when enabling GLSL mode with UV layers.

I can't reproduce issue here but crash is most likely caused by passing
a NULL pointer to glUniform2fv.

This is caused by OpenSubdiv changes to the codegen module, which pass
mtex layers to the uniform system
even when not needed.

Since Sergey is demoing OpenSubdiv in a few days, I'll
go with the easy fix for now which is just checking for NULL pointer, but
this needs to be fixed properly at some point.