MDEV-26249: Crash in Explain_node::print_explain_for_children with slow query log

The problem affected queries in form:

  SELECT FROM (SELECT where Split Materialized is applicable) WHERE 1=0

The problem was caused by this:
- The select in derived table uses two-phase optimization (due to a
  possible Split Materialized).
- The primary select has "Impossible where" and so it short-cuts its
  optimization.
- The optimization for the SELECT in the derived table is never finished,
  and EXPLAIN data structure has a dangling pointer to select #2.

Fixed with this: make JOIN::optimize_stage2() invoke optimization of
derived tables when it is handing a degenerate JOIN with zero tables.
We will not execute the derived tables but we need their query plans
for [SHOW]EXPLAIN.

mysql-test/main/explain_innodb.result

+#
+# MDEV-26249: Crash in in Explain_node::print_explain_for_children while writing to the slow query log
+#
+set @sql_tmp=@@slow_query_log;
+SET GLOBAL slow_query_log = 1;
+SET long_query_time = 0.000000;
+SET log_slow_verbosity = 'explain';
+CREATE TABLE t1 ( id varchar(50), KEY (id)) engine=innodb;
+SELECT * FROM (SELECT id FROM t1 GROUP BY id) dt WHERE 1=0;
+id
+select 1;
+1
+1
+explain
+SELECT * FROM (SELECT id FROM t1 GROUP BY id) dt WHERE 1=0;
+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	DERIVED	t1	index	NULL	id	53	NULL	1	Using index
+SET GLOBAL slow_query_log = @sql_tmp;
+drop table t1;

mysql-test/main/explain_innodb.test

+--echo #
+--echo # MDEV-26249: Crash in in Explain_node::print_explain_for_children while writing to the slow query log
+--echo #
+
+--source include/have_innodb.inc
+
+set @sql_tmp=@@slow_query_log;
+SET GLOBAL slow_query_log = 1;
+SET long_query_time = 0.000000;
+SET log_slow_verbosity = 'explain';
+
+CREATE TABLE t1 ( id varchar(50), KEY (id)) engine=innodb;
+SELECT * FROM (SELECT id FROM t1 GROUP BY id) dt WHERE 1=0;
+select 1;
+
+explain
+SELECT * FROM (SELECT id FROM t1 GROUP BY id) dt WHERE 1=0;
+
+SET GLOBAL slow_query_log = @sql_tmp;
+drop table t1;

sql/sql_select.cc

@@ -2732,6 +2732,14 @@ int JOIN::optimize_stage2()
     }
     if (make_aggr_tables_info())
       DBUG_RETURN(1);
+
+    /*
+      It could be that we've only done optimization stage 1 for
+      some of the derived tables, and never did stage 2.
+      Do it now, otherwise Explain data structure will not be complete.
+    */
+    if (select_lex->handle_derived(thd->lex, DT_OPTIMIZE))
+      DBUG_RETURN(1);
   }
   /*
     Even with zero matching rows, subqueries in the HAVING clause may