Set level pointers to NULL in topology_clear() to ease debugging.
Set level pointers to NULL in topology_clear() to ease debugging.

This commit fixes #51.



git-svn-id: 14be032f8f42541b1a281b51ae8ea69814daf20e@3949 4b44e086-7f34-40ce-a3bd-00e031736276

BlueBrain/hwloc,BlueBrain/hwloc,BlueBrain/hwloc,BlueBrain/hwloc