When using opengl attributes such as gl_VertexID in a shader its
location is set to -1. This location was used without checking in an
attribute array. This fails big time when called from python. For
example `print(shader.format_calc())` made python crash immediately.
this fixes https://github.com/JacquesLucke/animation_nodes/issues/1141
Reviewed By: fclem, brecht
Differential Revision: https://developer.blender.org/D5231
576963d7c1