Fix LP BUG#715034

Analysis:
The failed assert is a result of calling Item_sum_distinct::clear()
on an incomplete object for which Item_sum_distinct::setup() was
not yet called.

The reason is that JOIN::exec for the outer query calls JOIN::reinit()
for all its subqueries, which in turn calls clear() for all aggregate
functions of the subqueries. The call stack is:
mysql_explain_union -> mysql_select -> JOIN::exec -> select_desribe ->
mysql_explain_union -> mysql_select -> JOIN::reinit

This assert doesn't fail in the main 5.3 because constant subqueries
are being executed during the optimize phase of the outer query,
thus the Unique object is created before calling JOIN::exec for the
outer query, and Item_sum_distinct::clear() actually cleans the
Unique object.

Solution:
The best solution is the obvious one - substitute the assert with
a test whether Item_sum_distinct::tree is NULL.

mysql-test/r/subselect_mat_cost.result

@@ -3915,3 +3915,24 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 2	SUBQUERY	t1	system	NULL	NULL	NULL	NULL	1	
 3	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	Using temporary
 drop table t1, t2;
+#
+# LP BUG#715034 Item_sum_distinct::clear(): Assertion `tree != 0' failed
+#
+CREATE TABLE t2 ( f2 int(11)) ;
+CREATE TABLE t1 ( f3 int(11), KEY (f3)) ;
+INSERT INTO t1 VALUES (6),(4);
+EXPLAIN
+SELECT * FROM (SELECT * FROM t2) AS a2
+WHERE (SELECT distinct SUM(distinct f3 ) FROM t1);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+3	SUBQUERY	t1	index	NULL	f3	5	NULL	2	Using index
+2	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+insert into t2 values (1),(2);
+EXPLAIN
+SELECT * FROM (SELECT * FROM t2) AS a2
+WHERE (SELECT distinct SUM(distinct f3 ) FROM t1);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	2	
+3	SUBQUERY	t1	index	NULL	f3	5	NULL	2	Using index
+2	DERIVED	t2	ALL	NULL	NULL	NULL	NULL	2	

mysql-test/t/subselect_mat_cost.test

@@ -367,3 +367,21 @@ EXPLAIN SELECT f2 FROM t3 WHERE (
 ) IS NULL ;
 
 drop table t1, t2;
+
+--echo #
+--echo # LP BUG#715034 Item_sum_distinct::clear(): Assertion `tree != 0' failed
+--echo #
+
+CREATE TABLE t2 ( f2 int(11)) ;
+
+CREATE TABLE t1 ( f3 int(11), KEY (f3)) ;
+INSERT INTO t1 VALUES (6),(4);
+
+EXPLAIN
+SELECT * FROM (SELECT * FROM t2) AS a2
+WHERE (SELECT distinct SUM(distinct f3 ) FROM t1);
+
+insert into t2 values (1),(2);
+EXPLAIN
+SELECT * FROM (SELECT * FROM t2) AS a2
+WHERE (SELECT distinct SUM(distinct f3 ) FROM t1);

sql/item_sum.cc

@@ -1050,9 +1050,10 @@ bool Item_sum_distinct::unique_walk_function(void *element)
 void Item_sum_distinct::clear()
 {
   DBUG_ENTER("Item_sum_distinct::clear");
-  DBUG_ASSERT(tree != 0);                        /* we always have a tree */
-  null_value= 1;
+  /* During EXPLAIN there is no tree because it is created during execution. */
+  if (tree != 0)
     tree->reset();
+  null_value= 1;
   is_evaluated= FALSE;
   DBUG_VOID_RETURN;
 }