Valgrind warning "Syscall param pwrite64(buf) points to uninitialised byte(s)" on OPTIMIZE with a partitioned table

==10915== Syscall param pwrite64(buf) points to uninitialised byte(s)

==10915==    at 0x5046A73: ??? (syscall-template.S:82)

==10915==    by 0xCAA659: my_pwrite (my_pread.c:145)

==10915==    by 0xC33C92: inline_mysql_file_pwrite (mysql_file.h:1201)

==10915==    by 0xC388E1: mi_state_info_write (mi_open.c:928)

==10915==    by 0xC1CECC: update_state_info (mi_check.c:4459)

==10915==    by 0xC091C6: ha_myisam::repair(THD*, st_handler_check_param&, bool) (ha_myisam.cc:1169)

==10915==    by 0xC08776: ha_myisam::optimize(THD*, st_ha_check_opt*) (ha_myisam.cc:1026)

==10915==    by 0x7C49EC: handler::ha_optimize(THD*, st_ha_check_opt*) (handler.cc:3512)

==10915==    by 0xC72BA0: handle_opt_part(THD*, st_ha_check_opt*, handler*, unsigned int) (ha_partition.cc:1054)

==10915==    by 0xC732DE: ha_partition::handle_opt_partitions(THD*, st_ha_check_opt*, unsigned int) (ha_partition.cc:1206)

==10915==    by 0xC72944: ha_partition::optimize(THD*, st_ha_check_opt*) (ha_partition.cc:927)

==10915==    by 0x7C49EC: handler::ha_optimize(THD*, st_ha_check_opt*) (handler.cc:3512)

==10915==    by 0x717B19: mysql_admin_table(THD*, TABLE_LIST*, st_ha_check_opt*, char const*, thr_lock_type, bool, bool, unsigned int, int (*)(THD*, TABLE_LIST*, st_ha_check_opt*), int (handler::*)(THD*, st_ha_check_opt*), int (*)(THD*, TABLE_LIST*)) (sql_admin.cc:643)

==10915==    by 0x7193C2: Optimize_table_statement::execute(THD*) (sql_admin.cc:1055)

==10915==    by 0x92FAE3: Alter_table_optimize_partition_statement::execute(THD*) (sql_partition_admin.cc:84)

==10915==    by 0x61245C: mysql_execute_command(THD*) (sql_parse.cc:4454)

==10915==  Address 0x40c6c6c is on thread 4's stack

CREATE TABLE t1 (a INT, KEY (a)) ENGINE=MyISAM

  PARTITION BY KEY(a) PARTITIONS 2;

INSERT INTO t1 VALUES (3),(5),(1),(2);

ALTER TABLE t1 DISABLE KEYS;

ALTER TABLE t1 OPTIMIZE PARTITION p0;