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.

sql/sql_select.cc

@@ -2732,6 +2732,14 @@ setup_subq_exit:
     }
     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