When loading from odt, table cells which are covered (due to merging of
cells) are replaced with an empty cell by
SwXMLTableContext::ReplaceWithEmptyCell.  However if there is a sequence
of cells covered from above then their replacements are accidentally
inserted in reverse order, which produces this infinite loop problem when
saving as doc.

The reverse ordering in SwXMLTableContext::ReplaceWithEmptyCell was because
the insert position came from SwXMLTableContext::GetPrevStartNode which was
very careful to skip previous covered cells.  However those cells have
already been replaced with an empty cell so they should not be skipped.

Change-Id: I6a022cd1490afa181dbc3e4b2d6ed4af3077b363
Reviewed-on: https://gerrit.libreoffice.org/4008Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>