MDEV-31085 Crash when processing multi-update using view with optimizer_trace on

This bug caused server crash when processing a multi-update statement that
used views if optimizer tracing was enabled.
The bug was introduced in the patch for MDEV-30539 that could incorrectly
detect the most top level selects of queries if views were used in them.

Approved by Oleksandr Byelkin <sanja@mariadb.com>

mysql-test/main/opt_trace.test

@@ -677,6 +677,25 @@ INSERT INTO t1 VALUES (0,0);
 SELECT a FROM t1 WHERE (a,b) in (SELECT @c,@d);
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-31085: multi-update using view with optimizer trace enabled
+--echo #
+
+SET SESSION optimizer_trace = 'enabled=on';
+
+CREATE TABLE t (a int, b int);
+CREATE VIEW v AS SELECT 1 AS c UNION SELECT 2 AS c;
+INSERT INTO t VALUES (0,4),(5,6);
+UPDATE t, v SET t.b = t.a, t.a = v.c WHERE v.c < t.a;
+SELECT * FROM information_schema.optimizer_trace;
+
+SELECT * FROM t;
+
+SET optimizer_trace=DEFAULT;
+
+DROP VIEW v;
+DROP TABLE t;
+
 --echo #
 --echo # End of 10.4 tests
 --echo #

sql/sql_select.cc

@@ -28050,7 +28050,7 @@ void st_select_lex::print_item_list(THD *thd, String *str,
     outer_select() can not be used here because it is for name resolution
     and will return NULL at any end of name resolution chain (view/derived)
   */
-  bool top_level= (get_master()->get_master() == 0);
+  bool top_level= (get_master() == &thd->lex->unit);
   List_iterator_fast<Item> it(item_list);
   Item *item;
   while ((item= it++))
@@ -28157,7 +28157,7 @@ void st_select_lex::print(THD *thd, String *str, enum_query_type query_type)
     return;
   }
 
-  bool top_level= (get_master()->get_master() == 0);
+  bool top_level= (get_master() == &thd->lex->unit);
   enum explainable_cmd_type sel_type= SELECT_CMD;
   if (top_level)
     sel_type= get_explainable_cmd_type(thd);