MDEV-13351: Server crashes in st_select_lex::set_explain_type upon UNION with window function

Make st_select_lex::set_explain_type() take into account that JOIN_TABs
it is traversing may be also post-join aggregation JOIN_TABs (which
have pos_in_table_list=NULL, etc).

mysql-test/r/win.result

@@ -3160,3 +3160,16 @@ SELECT ('bar',1) IN ( SELECT c, ROW_NUMBER() OVER (PARTITION BY c) FROM t1);
 ('bar',1) IN ( SELECT c, ROW_NUMBER() OVER (PARTITION BY c) FROM t1)
 0
 DROP TABLE t1;
+#
+# MDEV-13351: Server crashes in st_select_lex::set_explain_type upon UNION with window function 
+#
+CREATE TABLE t1 (i INT);
+INSERT INTO t1 VALUES (1),(2);
+SELECT Nth_value(i,1) OVER() FROM t1 
+UNION ALL 
+( SELECT Nth_value(i,2) OVER() FROM t1 LIMIT 0 )
+;
+Nth_value(i,1) OVER()
+1
+1
+DROP TABLE t1;

mysql-test/t/win.test

@@ -1943,3 +1943,14 @@ INSERT IGNORE INTO t1 VALUES ('foo');
 SELECT ('bar',1) IN ( SELECT c, ROW_NUMBER() OVER (PARTITION BY c) FROM t1);
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-13351: Server crashes in st_select_lex::set_explain_type upon UNION with window function 
+--echo #
+CREATE TABLE t1 (i INT);
+INSERT INTO t1 VALUES (1),(2);
+SELECT Nth_value(i,1) OVER() FROM t1 
+UNION ALL 
+( SELECT Nth_value(i,2) OVER() FROM t1 LIMIT 0 )
+;
+DROP TABLE t1;
+

sql/sql_lex.cc

@@ -4395,10 +4395,16 @@ void st_select_lex::set_explain_type(bool on_the_fly)
         if (join)
         {
           bool uses_cte= false;
-          for (JOIN_TAB *tab= first_explain_order_tab(join); tab;
-               tab= next_explain_order_tab(join, tab))
+          for (JOIN_TAB *tab= first_linear_tab(join, WITHOUT_BUSH_ROOTS,
+                                                     WITH_CONST_TABLES);
+               tab;
+               tab= next_linear_tab(join, tab, WITHOUT_BUSH_ROOTS))
           {
-            if (tab->table && tab->table->pos_in_table_list->with)
+            /*
+              pos_in_table_list=NULL for e.g. post-join aggregation JOIN_TABs.
+            */
+            if (tab->table && tab->table->pos_in_table_list &&
+                tab->table->pos_in_table_list->with)
             {
               uses_cte= true;
               break;