fdo#75642: Check the high position block correctly.

Change-Id: I25deabf69e78c6e0354e8a528ab88891a30f9aec
(cherry picked from commit c078d5fef98ba77751b7a9593691d88dfc1c9ba1)
Reviewed-on: https://gerrit.libreoffice.org/8573
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-on: https://gerrit.libreoffice.org/8576
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Reviewed-by: Eike Rathke <erack@redhat.com>