fix discovered such bad thing in:
create table t3 (a int);
select * from (select 1 as a) as t1 join (select * from test.t3) as t2;
drop table t3;

we have:
(gdb) p all_tables->select_lex
$4 = (st_select_lex *) 0x1c1aae8
(gdb) p all_tables->next_global->next_global->select_lex
$5 = (st_select_lex *) 0x1c1aae8
(gdb) p all_tables->alias
$6 = 0x1c9dc18 "t1"
(gdb) p all_tables->next_global->next_global->alias
$7 = 0x1c9e848 "t2"

i.e. two different derived tables refer to the same SELECT_LEX structure