The helper function is used in export code to convert UTF-16 internal
string representation to UTF-8 used in XML encoding. I suppose that all
strings here should be already valid UTF-16 (an invalid input should
have been validated at import/input stage). An invalid string at this
stage means a programming error in another part of code that breaks this
precondition, and should not be handled at export stage.

This effectively changes flags used in conversion from UTF-16 to UTF-8,
so that now RTL_UNICODETOTEXT_FLAGS_INVALID_ERROR is used instead of
RTL_UNICODETOTEXT_FLAGS_INVALID_DEFAULT, so that the result would be
truncated on invalid bytes instead of inserting U+FFFD, and would fail
assertion in debug builds.
Other changed flags don't affect UTF-16-to-UTF-8 conversion.

Change-Id: I8b154249e66ee32b531b8ab503e0222b9108d829
Reviewed-on: https://gerrit.libreoffice.org/69830Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>