Deleting in-between pasting was causing the block iterators to become
invalid when the deletion took place, which resulted in a segfault.

Also cleaned up the bits about handling filtered rows.  It was using the
old row flag which is empty these days (so it never would have worked).
Plus we do bail out earlier if the destination contains a filtered row.

Change-Id: I78d3ee8bce1647c1c8685f92a3df3c38cc014811