(gdb) frame 1
#1 0x000000000069a948 in execute_sqlcom_select (thd=0x1b51408, all_tables=0x1bd0db0) at sql_parse.cc:5196
(gdb) p all_tables->alias
$1 = 0x1bd0d70 "t1"
(gdb) p all_tables->select_lex
$2 = (st_select_lex *) 0x1b53c28
(gdb) p all_tables->next_global->next_global->alias
$3 = 0x1bd1c28 "t2"
(gdb) p all_tables->next_global->next_global->select_lex
$4 = (st_select_lex *) 0x1b53c28
(gdb) p thd->query()
$5 = 0x1bcfc70 "select * from (select 1 as a) as t1 join (select * from test.t3) as t2"
(gdb)

the test suite is:
create table t3 (a int);
select * from (select 1 as a) as t1 join (select * from test.t3) as t2;
drop table t3;

Opened bug https://bugs.launchpad.net/maria/+bug/914224 to inform customers.
This bug blocks fixing https://mariadb.atlassian.net/browse/MDEV-64

Probably it is problem of parsing derived queries without tables.