Refactor Xdr node reading code slightly
Refactor Xdr node reading code slightly

* Move declaration of unique_id data buffers inside guards
* Fix assert
* Scope brackets around unique_id writing code
* Move reset code inside if (n_node_integers) block
* Fix comment

libMesh/libmesh,libMesh/libmesh,dschwen/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,libMesh/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,libMesh/libmesh,jwpeterson/libmesh,libMesh/libmesh,dschwen/libmesh,libMesh/libmesh,dschwen/libmesh,dschwen/libmesh,dschwen/libmesh,dschwen/libmesh,jwpeterson/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,libMesh/libmesh,dschwen/libmesh