EXPLAIN FORMAT=JSON crashes for loose scan query

create table ten(a int);

insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table t1 (a int, b int, c int, d int, key(a,b,c));

insert into t1 select  A.a, B.a, C.a, D.a from ten A, ten B, ten C, ten D;

MariaDB [j2]> explain select count(distinct b) from t1 group by a;

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

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

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

|    1 | SIMPLE      | t1    | range | NULL          | a    | 10      | NULL | 10250 | Using index for group-by (scanning) |

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

MariaDB [j2]> explain format=json select count(distinct b) from t1 group by a;

  mysqld: /home/psergey/dev-git/10.1-explain-analyze/sql/sql_explain.cc:1585: const char* Explain_quick_select::get_name_by_type(): Assertion `0' failed.

  Program received signal SIGABRT, Aborted.

  [Switching to Thread 0x7ffff0339700 (LWP 22858)]

  0x00007ffff5f90425 in __GI_raise (sig=<optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64

  64	../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.

(gdb)  wher

  #0  0x00007ffff5f90425 in __GI_raise (sig=<optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64

  #1  0x00007ffff5f93b8b in __GI_abort () at abort.c:91

  #2  0x00007ffff5f890ee in __assert_fail_base (fmt=<optimized out>, assertion=0x555556377658 "0", file=0x5555563777a8 "/home/psergey/dev-git/10.1-explain-analyze/sql/sql_explain.cc", line=<optimized out>, function=<optimized out>) at assert.c:94

  #3  0x00007ffff5f89192 in __GI___assert_fail (assertion=0x555556377658 "0", file=0x5555563777a8 "/home/psergey/dev-git/10.1-explain-analyze/sql/sql_explain.cc", line=1585, function=0x555556377fc0 "const char* Explain_quick_select::get_name_by_type()") at assert.c:103

  #4  0x0000555555b7b456 in Explain_quick_select::get_name_by_type (this=0x7fff9000a640) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_explain.cc:1585

  #5  0x0000555555b7b340 in Explain_quick_select::print_extra_recursive (this=0x7fff9000a640, str=0x7ffff0336d00) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_explain.cc:1554

  #6  0x0000555555b79019 in Explain_table_access::fill_key_str (this=0x7fff9000a340, key_str=0x7ffff0336e20, is_json=true) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_explain.cc:822

  #7  0x0000555555b7a405 in Explain_table_access::print_explain_json (this=0x7fff9000a340, query=0x7fff90009b90, writer=0x7ffff0337010, is_analyze=false) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_explain.cc:1230

  #8  0x0000555555b78df2 in Explain_basic_join::print_explain_json_interns (this=0x7fff90009d00, query=0x7fff90009b90, writer=0x7ffff0337010, is_analyze=false) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_explain.cc:777

  #9  0x0000555555b78cae in Explain_select::print_explain_json (this=0x7fff90009d00, query=0x7fff90009b90, writer=0x7ffff0337010, is_analyze=false) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_explain.cc:747

  #10 0x0000555555b770b0 in Explain_query::print_explain_json (this=0x7fff90009b90, output=0x7fff90008130, is_analyze=false) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_explain.cc:208

  #11 0x0000555555a1fece in execute_sqlcom_select (thd=0x5555582cac60, all_tables=0x7fff90007918) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_parse.cc:5733

  #12 0x0000555555a16556 in mysql_execute_command (thd=0x5555582cac60) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_parse.cc:2926

  #13 0x0000555555a237cb in mysql_parse (thd=0x5555582cac60, rawbuf=0x7fff90007558 "explain format=json select count(distinct b) from t1 group by a", length=63, parser_state=0x7ffff03380c0) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_parse.cc:7165

  #14 0x0000555555a127f5 in dispatch_command (command=COM_QUERY, thd=0x5555582cac60, packet=0x5555582d1ee1 "", packet_length=63) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_parse.cc:1462

  #15 0x0000555555a115bf in do_command (thd=0x5555582cac60) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_parse.cc:1090

  #16 0x0000555555b3e6fc in do_handle_one_connection (thd_arg=0x5555582cac60) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_connect.cc:1347

  #17 0x0000555555b3e441 in handle_one_connection (arg=0x5555582cac60) at /home/psergey/dev-git/10.1-explain-analyze/sql/sql_connect.cc:1258

  #18 0x0000555555edae78 in pfs_spawn_thread (arg=0x5555582d71c0) at /home/psergey/dev-git/10.1-explain-analyze/storage/perfschema/pfs.cc:1860

  #19 0x00007ffff691de9a in start_thread (arg=0x7ffff0339700) at pthread_create.c:308

  #20 0x00007ffff604e3fd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112