We no longer use parent() for interior_parent()
We no longer use parent() for interior_parent()

Fixing this fixes a segfault in a unit test for me.  Perhaps this isn't
triggering on CI because we've still got some backwards compatibility
without --disable-deprecated configured?

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