fdo#77806: Use the common block position set for start and end listening.

Else an invalid iterator may result, which would eventually leads to a
crash.

Change-Id: Ie909de61244d661c72a3450cc69b29fbb218a248
(cherry picked from commit 7fbe0f56192f7e106c560646d37fbb93b69b0446)
Reviewed-on: https://gerrit.libreoffice.org/9225
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>