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.test

@@ -0,0 +1,20 @@
+--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;