Fix for LibreOffice crash during Document RoundTrip

Description: Some Documents were crashing LO During Roundtrip.
This was because of setting wrong gridSpan value during export.

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

Change-Id: Ibcb6d4f00520500a98c16eb40dec3b052ad6b3f9
Reviewed-on: https://gerrit.libreoffice.org/6672
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>