Fix DEBUG-mode segfault
Fix DEBUG-mode segfault

When using the new "call dof_indices with elem=NULL" behavior,
dof_indices really can't dereference elem...

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