...that would implicitly be sign extended (for plain char only if it is signed),
so non-ASCII char values would trigger the isUnicodeCodePoint assert.

Change-Id: Iaf8024ad509e64525558e882fe3fd078cfb4ea91
Reviewed-on: https://gerrit.libreoffice.org/35523Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>