fdo#86687 Avoid deadlock updating progress bar

Change-Id: I13f93372fedc67bfd25d36a5bee10a004fe9f8f6
Reviewed-on: https://gerrit.libreoffice.org/13147
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>