These helpers are used in code generator code; so any invalid UTF-8
or UTF-16 in the conversion is programmer's error which needs fixing.
Thus, the behavior of toUtf8/fromUtf8 which asserts the validity is
fine here.

Change-Id: I3004e233c9de59f8e348455f1f04d23e8c51ed3d
Reviewed-on: https://gerrit.libreoffice.org/70249
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>