bugfix: kern_row_to_column assumed all the columns in kcs are part of row-store

gpusort uses internal columns on the last and second last column; not to be translated
from the row-store. So, we add a marker on attreferenced[i]; that indicates all the
referenced columns in row-store are already fetched, if attreferenced[i] is negative.
fixbug: do_insert_tuple() didn't check NULL values