signal 7 after calling stored procedure, that uses regexp

DELIMITER $$;

CREATE PROCEDURE p1(m1 varchar(5), m2 varchar(5))

BEGIN

SELECT a FROM

	(SELECT "aa" a) t

	JOIN (SELECT "aa" b) t1 on t.a=t1.b

WHERE t.a regexp m1 and t1.b regexp m2

GROUP BY a;

END$$

DELIMITER ;$$

CALL p1('a','a');

main.1_my                                [ fail ]  Found warnings/errors in server log file!

        Test ended at 2018-06-28 14:48:30

line

Error: Freeing overrun buffer  mysys/safemalloc.c:194, mysys/my_malloc.c:219, sql/item_cmpfunc.h:2092, sql/item_cmpfunc.h:2113, sql/item.h:1842, sql/sql_class.cc:3457, sql/sql_class.cc:2079, sql/sp_head.cc:1345

Error: Freeing overrun buffer  mysys/safemalloc.c:194, mysys/my_malloc.c:219, sql/item_cmpfunc.h:2092, sql/item_cmpfunc.h:2113, sql/item.h:1842, sql/sql_class.cc:3457, sql/sql_class.cc:2079, sql/sp_head.cc:1345

^ Found warnings in /home/alice/git/10.2/mysql-test/var/log/mysqld.1.err

180628 14:27:36 [ERROR] mysqld got signal 7 ;

Server version: 10.3.7-MariaDB

key_buffer_size=134217728

read_buffer_size=131072

max_used_connections=1

max_threads=153

thread_count=7

It is possible that mysqld could use up to 

key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 467389 K  bytes of memory

Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x7f7c600009a8

Attempting backtrace. You can use the following information to find out

where mysqld died. If you see no messages after this, something went

terribly wrong...

stack_bottom = 0x7f7ccc247ec0 thread_stack 0x49000