Make old_dof_object assertion less zealous
Make old_dof_object assertion less zealous

The code that sets old_dof_object tries to only do so when there are
dofs to keep track of; our assertions should do likewise.

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