tests/xmlbuffer: Dont't free_xmlbuffer() after topology_destroy()
tests/xmlbuffer: Dont't free_xmlbuffer() after topology_destroy()

The topology parameter isn't used internally yet, but the user
doesn't know that, so he shouldn't pass a destroyed topology
to free_xmlbuffer().

Stolen from components branch commit r4739

git-svn-id: 14be032f8f42541b1a281b51ae8ea69814daf20e@4984 4b44e086-7f34-40ce-a3bd-00e031736276

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