Allow cloning of PetscVector with n=0
Allow cloning of PetscVector with n=0

This is almost certainly a red herring w.r.t. the bug I'm actually
hunting down, but it seems to be a bug in its own right - when we init
a vector we expect it to be initialized.

Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh