Server crashes in Explain_node::print_explain_for_children upon SHOW EXPLAIN on UNION

--let $con_select= `SELECT CONNECTION_ID()`

CREATE TABLE t1 (a INT, b INT, PRIMARY KEY (a));

INSERT INTO t1 VALUES (1,1),(2,2);

--connect (con1,localhost,root,,)

--let $con_explain= `SELECT CONNECTION_ID()`

--delimiter |

send_eval

    BEGIN NOT ATOMIC

    DECLARE CONTINUE HANDLER FOR 1933 BEGIN END;

    LOOP

      SHOW EXPLAIN FOR $con_select;

    END LOOP;

    END |

--delimiter ;

--connection default

--let $run= 1000

while ($run)

{

    SELECT a, b FROM t1 UNION ( SELECT a, b FROM ( SELECT a, COUNT(*) AS b FROM t1 GROUP BY a ) AS sq LIMIT 0 );

    --dec $run

}

eval KILL $con_explain;

# Cleanup

DROP TABLE t1;

#3  <signal handler called>

#4  0x000055a271233448 in Explain_node::print_explain_for_children (this=0x7fb68016fa80, query=0x7fb6800178b0, output=0x7fb674049348, explain_flags=0 '\000', is_analyze=false) at /data/src/10.3/sql/sql_explain.cc:640

#5  0x000055a271233cc1 in Explain_select::print_explain (this=0x7fb68016fa80, query=0x7fb6800178b0, output=0x7fb674049348, explain_flags=0 '\000', is_analyze=false) at /data/src/10.3/sql/sql_explain.cc:824

#6  0x000055a271232b20 in Explain_union::print_explain (this=0x7fb68016fb98, query=0x7fb6800178b0, output=0x7fb674049348, explain_flags=0 '\000', is_analyze=false) at /data/src/10.3/sql/sql_explain.cc:487

#7  0x000055a2712317c5 in Explain_query::print_explain (this=0x7fb6800178b0, output=0x7fb674049348, explain_flags=0 '\000', is_analyze=false) at /data/src/10.3/sql/sql_explain.cc:208

#8  0x000055a27105f5ba in LEX::print_explain (this=0x7fb680004908, output=0x7fb674049348, explain_flags=0 '\000', is_analyze=false, printed_anything=0x7fb690d1637f) at /data/src/10.3/sql/sql_lex.cc:4922

#9  0x000055a271129297 in Show_explain_request::call_in_target_thread (this=0x7fb690cca330) at /data/src/10.3/sql/sql_show.cc:2987

#10 0x000055a2712645e8 in Apc_target::process_apc_requests (this=0x7fb6800048b8) at /data/src/10.3/sql/my_apc.cc:222

#11 0x000055a27102dea5 in THD::check_killed (this=0x7fb680000b00, dont_send_error_message=false) at /data/src/10.3/sql/sql_class.h:3063

#12 0x000055a2711004ed in evaluate_join_record (join=0x7fb680015cc8, join_tab=0x7fb68016e118, error=0) at /data/src/10.3/sql/sql_select.cc:19613

#13 0x000055a27110031d in sub_select (join=0x7fb680015cc8, join_tab=0x7fb68016e118, end_of_records=false) at /data/src/10.3/sql/sql_select.cc:19566

#14 0x000055a2710ff5cf in do_select (join=0x7fb680015cc8, procedure=0x0) at /data/src/10.3/sql/sql_select.cc:19067

#15 0x000055a2710d64d7 in JOIN::exec_inner (this=0x7fb680015cc8) at /data/src/10.3/sql/sql_select.cc:4040

#16 0x000055a2710d58aa in JOIN::exec (this=0x7fb680015cc8) at /data/src/10.3/sql/sql_select.cc:3834

#17 0x000055a2711856fc in st_select_lex_unit::exec (this=0x7fb6800049d0) at /data/src/10.3/sql/sql_union.cc:1497

#18 0x000055a271180165 in mysql_union (thd=0x7fb680000b00, lex=0x7fb680004908, result=0x7fb680015bb8, unit=0x7fb6800049d0, setup_tables_done_option=0) at /data/src/10.3/sql/sql_union.cc:41

#19 0x000055a2710c854a in handle_select (thd=0x7fb680000b00, lex=0x7fb680004908, result=0x7fb680015bb8, setup_tables_done_option=0) at /data/src/10.3/sql/sql_select.cc:360

#20 0x000055a271090110 in execute_sqlcom_select (thd=0x7fb680000b00, all_tables=0x7fb680012bf0) at /data/src/10.3/sql/sql_parse.cc:6291

#21 0x000055a271086b45 in mysql_execute_command (thd=0x7fb680000b00) at /data/src/10.3/sql/sql_parse.cc:3821

#22 0x000055a27109441b in mysql_parse (thd=0x7fb680000b00, rawbuf=0x7fb680012818 "SELECT a, b FROM t1 UNION ( SELECT a, b FROM ( SELECT a, COUNT(*) AS b FROM t1 GROUP BY a ) AS sq LIMIT 0 )", length=107, parser_state=0x7fb690d175e0, is_com_multi=false, is_next_command=false) at /data/src/10.3/sql/sql_parse.cc:7834

#23 0x000055a271080f8d in dispatch_command (command=COM_QUERY, thd=0x7fb680000b00, packet=0x7fb68011de81 "SELECT a, b FROM t1 UNION ( SELECT a, b FROM ( SELECT a, COUNT(*) AS b FROM t1 GROUP BY a ) AS sq LIMIT 0 )", packet_length=107, is_com_multi=false, is_next_command=false) at /data/src/10.3/sql/sql_parse.cc:1856

#24 0x000055a27107f8d5 in do_command (thd=0x7fb680000b00) at /data/src/10.3/sql/sql_parse.cc:1401

#25 0x000055a2711f6894 in do_handle_one_connection (connect=0x55a274b69ab0) at /data/src/10.3/sql/sql_connect.cc:1403

#26 0x000055a2711f65f6 in handle_one_connection (arg=0x55a274b69ab0) at /data/src/10.3/sql/sql_connect.cc:1308

#27 0x000055a271ba4c2c in pfs_spawn_thread (arg=0x55a274ac7190) at /data/src/10.3/storage/perfschema/pfs.cc:1862

#28 0x00007fb69868e4a4 in start_thread (arg=0x7fb690d18700) at pthread_create.c:456

#29 0x00007fb696bd5d0f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97