MDEV-7038 Assertion `status_var.memory_used == 0' failed in THD::~THD() on disconnect after executing EXPLAIN for multi-table UPDATE

Added test case that caught this bug. It is no longer reproducible in
the current tree.

mysql-test/r/explain_non_select.result

@@ -219,3 +219,16 @@ OK
 OK
 drop function f1;
 drop table t1;
+#
+# MDEV-7038: Assertion `status_var.memory_used == 0' failed in THD::~THD()
+#            on disconnect after executing EXPLAIN for multi-table UPDATE
+#
+CREATE TABLE t1 (a INT);
+CREATE VIEW v1 AS SELECT * FROM t1;
+INSERT INTO t1 VALUES (1),(2);
+EXPLAIN UPDATE v1, mysql.user SET v1.a = v1.a + 1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	
+1	SIMPLE	user	index	NULL	PRIMARY	420	NULL	4	Using index
+DROP TABLE t1;
+DROP VIEW v1;

mysql-test/t/explain_non_select.test

@@ -197,4 +197,14 @@ select 'OK';
 drop function f1;
 drop table t1;
 
+--echo #
+--echo # MDEV-7038: Assertion `status_var.memory_used == 0' failed in THD::~THD()
+--echo #            on disconnect after executing EXPLAIN for multi-table UPDATE
+--echo #
 
+CREATE TABLE t1 (a INT);
+CREATE VIEW v1 AS SELECT * FROM t1;
+INSERT INTO t1 VALUES (1),(2);
+EXPLAIN UPDATE v1, mysql.user SET v1.a = v1.a + 1;
+DROP TABLE t1;
+DROP VIEW v1;