LP:460420 - MyISAM segfault in debug binary on ALTER TABLE

branch from maria-5.1-merge (sorry can't verify exact revid but it's from within last week)
BUILD/compile-pentium-debug-max-no-ndb
So it's a debug binary. run...

CREATE TABLE `actors_movies` (
`actor_id` int(10) unsigned NOT NULL,
`movie_id` int(10) unsigned NOT NULL,
UNIQUE KEY `actor_id` (`actor_id`,`movie_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

– table is empty
ALTER TABLE actors_movies ENGINE=MyISAM;

segfaults. With gdb attached we find the following:

Program received signal SIGSEGV, Segmentation fault.
0x00000000008911bc in ha_myisam::enable_indexes (this=Cannot access memory at address 0x7fd0dce20868
) at ha_myisam.cc:1444
1444 int ha_myisam::enable_indexes(uint mode)
(gdb) bt
#0 0x00000000008911bc in ha_myisam::enable_indexes (this=Cannot access memory at address 0x7fd0dce20868
) at ha_myisam.cc:1444
#1 0x000000000088bff8 in ha_myisam::end_bulk_insert (this=0x2a8be90, abort=false) at ha_myisam.cc:1610
#2 0x000000000071e108 in handler::ha_end_bulk_insert (this=0x2a8be90, abort=false) at handler.h:1242
#3 0x00000000007c88ac in copy_data_between_tables (from=0x30d9988, to=0x2a8adc8, create=@0x7fd0dce43a38, ignore=false, order_num=0, order=0x0, copied=0x7fd0dce42068, deleted=0x7fd0dce42060,
keys_onoff=LEAVE_AS_IS, error_if_not_empty=false) at sql_table.cc:7677
#4 0x00000000007d4d07 in mysql_alter_table (thd=0x7fd0c18b5298, new_db=0x29d05f8 "imdb", new_name=0x29d0280 "actors_movies", create_info=0x7fd0dce43420, table_list=0x29d02c8, alter_info=0x7fd0dce439f0,
order_num=0, order=0x0, ignore=false) at sql_table.cc:7087
#5 0x0000000000680aaf in mysql_execute_command (thd=0x7fd0c18b5298) at sql_parse.cc:2900
#6 0x0000000000687cdc in mysql_parse (thd=0x7fd0c18b5298, inBuf=0x29d01c0 "alter table actors_movies engine=myisam", length=39, found_semicolon=0x7fd0dce44c00) at sql_parse.cc:5979
#7 0x0000000000688ade in dispatch_command (command=COM_QUERY, thd=0x7fd0c18b5298, packet=0x7fd0c1961859 "alter table actors_movies engine=myisam", packet_length=39) at sql_parse.cc:1223
#8 0x0000000000689f4e in do_command (thd=0x7fd0c18b5298) at sql_parse.cc:862
#9 0x0000000000675a1c in handle_one_connection (arg=0x7fd0c18b5298) at sql_connect.cc:1130
#10 0x00007fd0dc90d3ba in start_thread () from /lib/libpthread.so.0
#11 0x00007fd0db980fcd in clone () from /lib/libc.so.6
#12 0x0000000000000000 in ?? ()

Again, note that this is a debug binary; since DEBUG has different structures, it's quite possible that this only happens in DEBUG binaries and not in regular. Nevertheless, it looks like a bug.