Fix mesh reference in VTKIO write
Fix mesh reference in VTKIO write

If the "writing" constructor is used, MeshInput has an empty pointer to mesh. Calling write_nodal_data then causes dereferencing null pointer. Should be fine to use the const mesh reference from MeshOutput.
dschwen/libmesh,jwpeterson/libmesh,libMesh/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,roystgnr/libmesh,dschwen/libmesh,roystgnr/libmesh,libMesh/libmesh,libMesh/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,dschwen/libmesh,roystgnr/libmesh,libMesh/libmesh,capitalaslash/libmesh,dschwen/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,dschwen/libmesh,jwpeterson/libmesh,dschwen/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,roystgnr/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,libMesh/libmesh,BalticPinguin/libmesh,dschwen/libmesh,libMesh/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,libMesh/libmesh,libMesh/libmesh,dschwen/libmesh,BalticPinguin/libmesh,roystgnr/libmesh