(related: fdo#78502) writerfilter: RTF import: fix invalid string copy

... in leveltext destination.

Change-Id: I74de6d14170130bf33923854a0c9851dc7cc390f
(cherry picked from commit b94bd40b915ab32d18d43fc60dfda932e4e00ca8)
Reviewed-on: https://gerrit.libreoffice.org/10397
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>