fdo#71784: Invalid Sections getting added to the document after RT

Description: There was a problem for some documents, during export Invalid sectPr were getting added.
The root cause was faulty calculation of PageDesc value.
This was the reason for increasing number of pages in RT doc.

Conflicts:
	sw/qa/extras/ooxmlexport/ooxmlexport.cxx

Change-Id: I0700c735545614730d26be187d9047fd20ebf134
Reviewed-on: https://gerrit.libreoffice.org/6813
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>