Optimization for unfiltered indexes was incorrect

The code in CKeyCursor::_lookup:

    if ((lastSeg == 0) && !matched)
    {
        //Special case reading a file with no filter - fall into the next processing.
        node.clear();
        matched = true;
    }

However lastSeg is inclusive - so an unfiltered index read should be comparing against -1.

However that value is never passed, and some of the functions that are called like
SegMonitorList::matchesBuffer will not process it correctly. NOTE: IndexRowFilter::matchesBuffer does support it.