BUG#752992: Wrong results for a subquery with 'semijoin=on'

- Let advance_sj_state() save the value of JOIN::cur_dups_producing_tables
  in POSITION::prefix_dups_producing_tables, and restore_sj_state() restore
  it.

sql/sql_select.h

@@ -573,6 +573,8 @@ typedef struct st_position
     semi-join's ON expression so we can correctly account for fanout.
   */
   table_map sjm_scan_need_tables;
+
+  table_map prefix_dups_producing_tables;
 } POSITION;