MDEV-23160: SIGSEGV in Explain_node::print_explain_for_children on UNION SELECT

and also MDEV-25564, MDEV-18157.

Attempt to produce EXPLAIN output caused a crash in
Explain_node::print_explain_for_children. The cause of this was that an
Explain_node (actually a derived) had a link to child select#N, but
there was no query plan present for select#N.

The query plan wasn't present because the subquery was eliminated.
- Either it was a degenerate subquery like "(SELECT 1)" in MDEV-25564.
- Or it was a subquery in a UNION subquery's ORDER BY clause:
   col IN (SELECT ... UNION
           SELECT ... ORDER BY (SELECT FROM t1))

In such cases, legacy code structure in subquery/union processing code(*)
makes it hard to detect that the subquery was eliminated, so we end up
with EXPLAIN data structures (Explain_node::children) having dangling
links to child subqueries.
Do make the checks and don't follow the dangling links.

(In ideal world, we should not have these dangling links. But fixing
the code (*) would have high risk for the stable versions).

mysql-test/main/explain.test

@@ -333,3 +333,34 @@ explain replace into t2 select 100, (select a from t1);
 drop table t1, t2;
 
 --echo # End of 10.1 tests
+
+--echo #
+--echo # End of 10.2 test
+--echo #
+
+--echo #
+--echo # MDEV-25564: Server crashed on running some EXPLAIN statements
+--echo #
+
+EXPLAIN (SELECT 1,3) UNION (SELECT 2,1) ORDER BY (SELECT 2);
+
+--echo #
+--echo # MDEV-23160: SIGSEGV in Explain_node::print_explain_for_children on UNION SELECT
+--echo #
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2),(3);
+
+EXPLAIN
+SELECT *
+FROM t1
+WHERE
+  a IN (SELECT a FROM t1
+        UNION
+        SELECT a FROM t1 ORDER BY (SELECT a))
+UNION
+  SELECT * FROM t1 ORDER BY (SELECT a);
+drop table t1;
+
+explain
+VALUES ( (VALUES (2))) UNION VALUES ( (SELECT 3));