address some out of bounds oddness seen with
rtl_ustr_getlength_heap_buffer_overflow.sample

Change-Id: I5a9772de9607644f43e74174f73053d292ca7cc0
Reviewed-on: https://gerrit.libreoffice.org/55722Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>