[Fixed in devtree] EXPLAIN crashes for a CTE query

create table t20 (a int, b int);

insert into t20 values (1,1);

explain with X as (select * from t20) select * from t20;

Program received signal SIGSEGV, Segmentation fault.

[Switching to Thread 0x7ffff4300f00 (LWP 18016)]

0x0000555555a75e8c in st_select_lex::update_used_tables (this=0x7fff54012dc8)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_lex.cc:4217

4217      join->result->update_used_tables();

(gdb) p join->result

$1 = (select_result *) 0x0

(gdb) wher

#0  0x0000555555a75e8c in st_select_lex::update_used_tables (this=0x7fff54012dc8)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_lex.cc:4217

#1  0x0000555555ac0024 in JOIN::optimize_inner (this=0x7fff54015780)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_select.cc:1155

#2  0x0000555555abfc2c in JOIN::optimize (this=0x7fff54015780)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_select.cc:1063

#3  0x0000555555ac7ed0 in mysql_select (thd=0x7fff54000b00, tables=0x7fff540139c0, wild_num=0, fields=..., conds=

    0x0, og_num=0, order=0x0, group=0x0, having=0x0, proc_param=0x0, select_options=2416184068, 

    result=0x7fff54014890, unit=0x7fff54013160, select_lex=0x7fff54012dc8)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_select.cc:3521

#4  0x0000555555afad29 in mysql_explain_union (thd=0x7fff54000b00, unit=0x7fff54013160, result=0x7fff54014890)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_select.cc:24511

#5  0x0000555555afaa8e in select_describe (join=0x7fff54014f10, need_tmp_table=false, need_order=false, 

    distinct=false, message=0x0) at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_select.cc:24466

#6  0x0000555555ac77a8 in JOIN::exec_inner (this=0x7fff54014f10)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_select.cc:3313

#7  0x0000555555ac6e78 in JOIN::exec (this=0x7fff54014f10)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_select.cc:3153

#8  0x0000555555ac7f60 in mysql_select (thd=0x7fff54000b00, tables=0x7fff54014198, wild_num=1, fields=..., 

    conds=0x0, og_num=0, order=0x0, group=0x0, having=0x0, proc_param=0x0, select_options=2147748612, 

    result=0x7fff54014890, unit=0x7fff54004550, select_lex=0x7fff54004c70)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_select.cc:3535

#9  0x0000555555afad29 in mysql_explain_union (thd=0x7fff54000b00, unit=0x7fff54004550, result=0x7fff54014890)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_select.cc:24511

#10 0x0000555555a8d305 in execute_sqlcom_select (thd=0x7fff54000b00, all_tables=0x7fff54014198)

    at /home/psergey/dev-git/10.2-window-funcs-r10/sql/sql_parse.cc:6238