Changes

nolibxml/import: make sure there's a \0 at the end of the buffer
If the user gives a non-NULL terminated buffer to set_xmlbuffer(), the
nolibxml() backend could go crazy. At least sscanf() and strchr() could
pass the end of the buffer.
Refs open-mpi/ompi#6181
Thanks to Gilles Gouaillardet for the patch.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

libxml/export_xmlbuffer: include the ending \0 in the returned length
It was omitted in the libxml case, even if that \0 actually exits.
And document things.
Refs open-mpi/ompi#6181
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

libxml/export_xmlbuffer: detect and handle libxml dump errors
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

tests/xmlbuffer: really check nolibxml/libxml import/export
LIBXML env vars are parsed only once per process, we cannot change them
between imports/exports.
wrapper.sh now runs xmlbuffer once per combination of env vars.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

tests/xmlbuffer: check that the ending \0 is inside the returned length
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

nolibxml/import: don't duplicate the input buffer
Not needed since we don't support multiple loads/reloads anymore.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>