- in advance_sj_state(), remember join->cur_dups_producing_tables in 
  pos->prefix_dups_producing_tables *before* we modify it, so that 
  restore_prev_sj_state() restores cur_dups_producing_tables in all cases.
- Updated test results in subselect_sj2[_jcl6].result (the original EXPLAIN
  was invalid there)