handle invalid xml files in orcus, fdo#63844

Change-Id: I82fff6ab89acece0e46c92bfca2c7faf967639b8
Signed-off-by: Kohei Yoshida <kohei.yoshida@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/7077
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>