Add a cheap check for this in both SaxWriter and FastSaxSerializer
so we can find such bugs earlier, e.g. with the weekly crashtesting.

Don't do a correct check but a cheap & fast one, let's ignore
non-ASCII characters for now as the only filter with such is UOF
and that is implemented with XSLT, not this sax code.

Change-Id: I4db8f70ffb23684d4cb4211468519edd6c7c465f
Reviewed-on: https://gerrit.libreoffice.org/50507Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>