Bugfix for mismatched id sizes when reading xdr

This fixes a regression in our BuildBot "BigIDs" tests.  When
sizeof(T) is different than sizeof(dof_id_type) (as happened when an
8-byte-dof_id libMesh reads a file written with a 4-byte-dof_id
libMesh), we need to use the former rather than the latter to decide
what the code is for "no parent element".