Fixed 2 valgrind bugs in the tree for mwl #106.

One was due the fact that the field SELECT_LEX::insert_tables was not
initialized while the second was due to missing initialization of
JOIN_TAB::preread_init_done in subselect_hash_sj_engine::make_unique_engine. 
Removed an invalid assert.

sql/item_subselect.cc

@@ -4165,6 +4165,7 @@ subselect_hash_sj_engine::make_unique_engine()
     DBUG_RETURN(NULL);
 
   tab->table= tmp_table;
+  tab->preread_init_done= FALSE;
   tab->ref.tmp_table_index_lookup_init(thd, tmp_key, it, FALSE);
 
   DBUG_RETURN(new subselect_uniquesubquery_engine(thd, tab, item,

sql/sql_lex.cc

@@ -1672,6 +1672,7 @@ void st_select_lex::init_select()
   cond_value= having_value= Item::COND_UNDEF;
   inner_refs_list.empty();
   full_group_by_flag= 0;
+  insert_tables= 0;
   merged_into= 0;
 }
 

sql/table.cc

@@ -3797,7 +3797,6 @@ bool TABLE_LIST::prep_check_option(THD *thd, uint8 check_opt_type)
 
     if (where)
     {
-      DBUG_ASSERT(where->fixed);
       check_option= where->copy_andor_structure(thd);
     }
     if (is_cascaded)