BUG#933407: Valgrind warnings in mark_as_null_row with materialization+semijoin, STRAIGHT_JOIN, impossible WHERE
- In return_zero_rows(), don't call mark_as_null_row() for semi-join 
  materialized tables, because 1) they may have been already freed, and 
  2)there is no real need to call mark_as_null_row() for them.