nullspace preconditioner memory overrun

[iaae]

I was following the example of the nullspace.cpp demo code and tested the effectiveness on a problem with about 1M equations. It crashed when trying to use components beyond the prm.precond.coarsening.nullspace.B vector (ndim=3, so 6 modes)

prm.precond.coarsening.nullspace.cols = amgcl::coarsening::rigid_body_modes( ndim, coo, prm.precond.coarsening.nullspace.B);

Thanks.