Server crashes with SIGSEGV in merge_buffers with low value for tmp_table_size and an aggregate function with the distinct clause

create table t1 (
a VARCHAR(1020),
b int
);
insert into t1 values
( 0 , 1 ),
( 1 , 2 ),
( 2 , 3 ),
( 3 , 4 ),
( 4 , 5 ),
( 5 , 6 ),
( 6 , 7 ),
( 7 , 8 ),
( 8 , 9 ),
( 9 , 10 ),
( 10 , 11 ),
( 11 , 12 ),
( 12 , 13 ),
( 13 , 14 ),
( 14 , 15 ),
( 15 , 16 ),
( 16 , 17 ),
( 17 , 18 ),
( 18 , 19 ),
( 25 , 26 ),
( 26 , 27 ),
( 9 , 120 ),
( 10 , 121 ),
( 11 , 122 ),
( 12 , 123 ),
( 13 , 124 ),
( 14 , 125 ),
( 15 , 126 ),
( 16 , 127 );
set tmp_table_size=1024;
select count(distinct a) from t1;
drop table t1;