MDEV-7927: Server crashes in in Time_and_counter_tracker::incr_loops

- JOIN::save_explain_data should also set JOIN::tracker (it already
  sets join_tab[0].tracker). This is needed for UNIONs inside subqueries.

mysql-test/r/explain_json.result

@@ -894,3 +894,15 @@ EXPLAIN
   }
 }
 DROP TABLE t1, t2;
+#
+# MDEV-7927: Server crashes in in Time_and_counter_tracker::incr_loops
+#
+CREATE TABLE t1 (i INT);
+INSERT INTO t1 VALUES (1),(2);
+EXPLAIN SELECT * FROM t1 WHERE 3 IN ( SELECT 4 UNION SELECT 5 );
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+3	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+DROP TABLE t1;

mysql-test/t/explain_json.test

@@ -200,3 +200,11 @@ INSERT INTO t2 VALUES (3),(4);
 EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE a <> ALL ( SELECT b FROM t2 );
 DROP TABLE t1, t2;
 
+--echo #
+--echo # MDEV-7927: Server crashes in in Time_and_counter_tracker::incr_loops
+--echo #
+CREATE TABLE t1 (i INT);
+INSERT INTO t1 VALUES (1),(2);
+EXPLAIN SELECT * FROM t1 WHERE 3 IN ( SELECT 4 UNION SELECT 5 );
+DROP TABLE t1;
+

sql/sql_select.cc

@@ -2376,8 +2376,10 @@ void JOIN::save_explain_data(Explain_query *output, bool can_overwrite,
       This is fake_select_lex. It has no query plan, but we need to set up a
       tracker for ANALYZE 
     */
-    Explain_union *eu= output->get_union(select_lex->master_unit()->first_select()->select_number);
+    uint nr= select_lex->master_unit()->first_select()->select_number;
+    Explain_union *eu= output->get_union(nr);
     join_tab[0].tracker= eu->get_fake_select_lex_tracker();
+    tracker= &eu->time_tracker;
   }
 }