Fixed -Wnull-pointer-arithmetic warning
Warning was:
VTK/Rendering/OpenGL2/vtkOpenGLVertexArrayObject.cxx:182:33: warning: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic] BUFFER_OFFSET(attrIt->Offset + attrIt->Stride*i/attrIt->Size)); ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~