Ensure that a parsed document that contains a malformed xmlns tag
doesn't cause a segfault when serializing it through the
XSAXSerializable interface.

Change-Id: I00811494b37a307711c1a7a86403a56da8b23776
Reviewed-on: https://gerrit.libreoffice.org/6004Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>