Fix buildbot failure in show_explain.test:

mysqltest: At line 477: query 'show explain for $thr2' failed: 1933: Target is not running an EXPLAINable command

After the fix for MDEV-4144, subquery with WHERE pk= (select ...) became a degenerate, constant 
SELECT.  It is not executed in normal way anymore, so it is not possible to catch it in-execution.

mysql-test/r/show_explain.result

@@ -442,9 +442,9 @@ WHERE pk= (SELECT MAX(pk) FROM t2 WHERE pk = 3)
 );
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	20	Using where
-2	SUBQUERY	t2	const	PRIMARY	PRIMARY	4	const	1	Using where
+2	SUBQUERY	t2	const	PRIMARY	PRIMARY	4	const	1	
 3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
-set @show_explain_probe_select_id=2;
+set @show_explain_probe_select_id=1;
 set debug_dbug='+d,show_explain_probe_do_select';
 SELECT * FROM t2 WHERE a = 
 (SELECT MAX(a) FROM t2 
@@ -453,7 +453,7 @@ WHERE pk= (SELECT MAX(pk) FROM t2 WHERE pk = 3)
 show explain for $thr2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	20	Using where
-2	SUBQUERY	t2	const	PRIMARY	PRIMARY	4	const	1	Using where
+2	SUBQUERY	t2	const	PRIMARY	PRIMARY	4	const	1	
 3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
 Warnings:
 Note	1003	SELECT * FROM t2 WHERE a = 

mysql-test/t/show_explain.test

@@ -466,7 +466,7 @@ explain SELECT * FROM t2 WHERE a =
         WHERE pk= (SELECT MAX(pk) FROM t2 WHERE pk = 3)
        );
 
-set @show_explain_probe_select_id=2;
+set @show_explain_probe_select_id=1;
 set debug_dbug='+d,show_explain_probe_do_select';
 send SELECT * FROM t2 WHERE a = 
        (SELECT MAX(a) FROM t2