fixed joincleunup to avoid double deletin tables, and too earlyfull cleanup in case of EXPLAIN

fixed cleunup of TMP_TABLE_PARAM
(BUG#6406)

mysql-test/r/subselect.result

@@ -1991,3 +1991,15 @@ ac
 NULL
 drop tables t1,t2;
 set @got_val= (SELECT 1 FROM (SELECT 'A' as my_col) as T1 ) ;
+create table t1 (a int, b int);
+create table t2 (a int, b int);
+insert into t1 values (1,1),(1,2),(1,3),(2,4),(2,5);
+insert into t2 values (1,3),(2,1);
+select distinct a,b, (select max(b) from t2 where t1.b=t2.a) from t1 order by t1.b;
+a	b	(select max(b) from t2 where t1.b=t2.a)
+1	1	3
+1	2	1
+1	3	NULL
+2	4	NULL
+2	5	NULL
+drop table t1, t2;

mysql-test/t/subselect.test

@@ -1289,3 +1289,13 @@ drop tables t1,t2;
 connect (root,localhost,root,,test,$MASTER_MYPORT,$MASTER_MYSOCK);
 connection root;
 set @got_val= (SELECT 1 FROM (SELECT 'A' as my_col) as T1 ) ;
+
+#
+# primary query with temporary table and subquery with groupping
+#
+create table t1 (a int, b int);
+create table t2 (a int, b int);
+insert into t1 values (1,1),(1,2),(1,3),(2,4),(2,5);
+insert into t2 values (1,3),(2,1);
+select distinct a,b, (select max(b) from t2 where t1.b=t2.a) from t1 order by t1.b;
+drop table t1, t2;

sql/sql_class.h

@@ -1305,7 +1305,7 @@ class TMP_TABLE_PARAM :public Sql_alloc
     if (copy_field)				/* Fix for Intel compiler */
     {
       delete [] copy_field;
-      copy_field=0;
+      save_copy_field= copy_field= 0;
     }
   }
 };

sql/sql_select.cc

@@ -936,7 +936,7 @@ JOIN::optimize()
       }
     }
     
-    if (select_lex->master_unit()->uncacheable)
+    if (select_lex->uncacheable)
     {
       if (!(tmp_join= (JOIN*)thd->alloc(sizeof(JOIN))))
 	DBUG_RETURN(-1);
@@ -3833,7 +3833,8 @@ JOIN::join_free(bool full)
   JOIN_TAB *tab,*end;
   DBUG_ENTER("JOIN::join_free");
 
-  full= full || !select_lex->uncacheable;
+  full= full || (!select_lex->uncacheable &&
+                 !thd->lex->describe);
 
   if (table)
   {
@@ -3862,6 +3863,7 @@ JOIN::join_free(bool full)
       for (tab= join_tab, end= tab+tables; tab != end; tab++)
 	tab->cleanup();
       table= 0;
+      tables= 0;
     }
     else
     {