Crash with partitioned table, PARTITION syntax, and index_merge

create table t1 (

  pk int primary key,

  a int,

  b int,

  filler char(32),

  key (a),

  key (b)

) engine=myisam  partition by range(pk) (

  partition p0 values less than (10),

  partition p1 values less than MAXVALUE

) ;

insert into t1 select 

   seq,

   MOD(seq, 100),

   MOD(seq, 100),

   'filler-data-filler-data'

from

 seq_1_to_50000;

explain 

select * from t1 partition (p1) where a=10 and b=10;

flush tables;

select * from t1 partition (p1)where a=10 and b=10;

MariaDB [j3]> explain 

    -> select * from t1 partition (p1) where a=10 and b=10;

+------+-------------+-------+-------------+---------------+------+---------+------+------+-----------------------------------+

| id   | select_type | table | type        | possible_keys | key  | key_len | ref  | rows | Extra                             |

+------+-------------+-------+-------------+---------------+------+---------+------+------+-----------------------------------+

|    1 | SIMPLE      | t1    | index_merge | a,b           | a,b  | 5,5     | NULL |    4 | Using intersect(a,b); Using where |

+------+-------------+-------+-------------+---------------+------+---------+------+------+-----------------------------------+

  Thread 38 "mysqld" received signal SIGSEGV, Segmentation fault.

  [Switching to LWP 803]

  0x0000555556776031 in ha_partition::compare_number_of_records (me=0x7ffe54076f10, a=0x7ffe54069c30, b=0x7ffe54069c34) at /home/psergey/dev-git/10.3.20/sql/ha_partition.cc:8058

(gdb) wher

  #0  0x0000555556776031 in ha_partition::compare_number_of_records (me=0x7ffe54076f10, a=0x7ffe54069c30, b=0x7ffe54069c34) at /home/psergey/dev-git/10.3.20/sql/ha_partition.cc:8058

  #1  0x00005555567a09e8 in my_qsort2 (base_ptr=0x7ffe54069c30, count=2, size=4, cmp=0x555556775ff8 <ha_partition::compare_number_of_records(ha_partition*, unsigned int const*, unsigned int const*)>, cmp_argument=0x7ffe54076f10) at /home/psergey/dev-git/10.3.20/mysys/mf_qsort.c:131

  #2  0x0000555556776a07 in ha_partition::info (this=0x7ffe54076f10, flag=280) at /home/psergey/dev-git/10.3.20/sql/ha_partition.cc:8340

  #3  0x0000555556769806 in ha_partition::open (this=0x7ffe54076f10, name=0x7ffe54044b80 "./j2/t1", mode=33, test_if_locked=1026) at /home/psergey/dev-git/10.3.20/sql/ha_partition.cc:3653

  #4  0x0000555555f4481d in handler::ha_open (this=0x7ffe54076f10, table_arg=0x7ffe54040a40, name=0x7ffe54044b80 "./j2/t1", mode=33, test_if_locked=1026, mem_root=0x0, partitions_to_open=0x0) at /home/psergey/dev-git/10.3.20/sql/handler.cc:2760

  #5  0x0000555556769acd in ha_partition::clone (this=0x7ffe5407c978, name=0x7ffe54044b80 "./j2/t1", mem_root=0x7ffe5403b510) at /home/psergey/dev-git/10.3.20/sql/ha_partition.cc:3752

  #6  0x00005555560acefe in QUICK_RANGE_SELECT::init_ror_merged_scan (this=0x7ffe6c014810, reuse_handler=false, local_alloc=0x7ffe5403b510) at /home/psergey/dev-git/10.3.20/sql/opt_range.cc:1503

  #7  0x00005555560ad3b0 in QUICK_ROR_INTERSECT_SELECT::init_ror_merged_scan (this=0x7ffe5403b4b0, reuse_handler=true, local_alloc=0x7ffe5403b510) at /home/psergey/dev-git/10.3.20/sql/opt_range.cc:1608

  #8  0x00005555560ad63e in QUICK_ROR_INTERSECT_SELECT::reset (this=0x7ffe5403b4b0) at /home/psergey/dev-git/10.3.20/sql/opt_range.cc:1643

  #9  0x0000555555ca281b in join_init_read_record (tab=0x7ffe54017460) at /home/psergey/dev-git/10.3.20/sql/sql_select.cc:20451

  #10 0x0000555555ca050b in sub_select (join=0x7ffe54014c88, join_tab=0x7ffe54017460, end_of_records=false) at /home/psergey/dev-git/10.3.20/sql/sql_select.cc:19527

  #11 0x0000555555c9f9f7 in do_select (join=0x7ffe54014c88, procedure=0x0) at /home/psergey/dev-git/10.3.20/sql/sql_select.cc:19070

  #12 0x0000555555c76a00 in JOIN::exec_inner (this=0x7ffe54014c88) at /home/psergey/dev-git/10.3.20/sql/sql_select.cc:4040

  #13 0x0000555555c75dd0 in JOIN::exec (this=0x7ffe54014c88) at /home/psergey/dev-git/10.3.20/sql/sql_select.cc:3834