In RTF, it's possible to start a cells merge using \clmgf, and simply
omit following cells in the row - they must merge automatically.
This makes HorizontallyMergedCell::m_nLastCol/Row uninitialized.
Previously, the uninitialized values arrived as 0,0 - thus the first
range's cell got merged with cell 0,0.

This change prevents the merge; in scenario above, absence of additional
cells in row will create merged cell automatically.

Change-Id: I68b84b7ec70d9512c541a077689369fa4a8dc0c5
Reviewed-on: https://gerrit.libreoffice.org/34079Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>