And fix
  ScXMLCachedRowAttrAccess::Cache
which was never setting its mnTab field, and hence would never
be hit.

And fix oox::xls::CellBlockBuffer, which was never setting mnCurrRow.

Change-Id: I2c46aa050b9ebe3c2dc2e52579555f97945dd61c
Reviewed-on: https://gerrit.libreoffice.org/61772
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>