qobject_from_jsonf() aborts on error, unlike qobject_from_jsonv(),
which returns null. Since all remaining users of qobject_from_jsonf()
cope fine with null, change it to return null.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-30-armbru@redhat.com>