nolibxml: check that the exported file was properly written
nolibxml: check that the exported file was properly written

Return an error if needed.

This also silences a warning about fwrite() return value being ignored.



git-svn-id: 14be032f8f42541b1a281b51ae8ea69814daf20e@4897 4b44e086-7f34-40ce-a3bd-00e031736276

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