nv50: avoid segfault with enabled but unbound vertex attrib
Before validating vertex arrays we need to check if a VBO is present. Checking if vb->buffer is not NULL fixes the issue. Fixes the following piglit test: gl-3.1-vao-broken-attrib Signed-off-by:Samuel Pitoiset <samuel.pitoiset@gmail.com> Reviewed-by:
Ilia Mirkin <imirkin@alum.mit.edu>