Valgrind error ha_maria::check_if_incompatible_data on altering a partitioned Aria table

[Elena Stepanova]

CREATE TABLE t1 (a INT, b INT, KEY(a)) ENGINE=Aria
  PARTITION BY KEY(a) PARTITIONS 2;
ALTER TABLE t1 ADD KEY (b);

==11035== Conditional jump or move depends on uninitialised value(s)
==11035== at 0x93EBA0: ha_maria::check_if_incompatible_data(st_ha_create_information*, unsigned int) (ha_maria.cc:3023)
==11035== by 0x7DACA3: ha_partition::check_if_incompatible_data(st_ha_create_information*, unsigned int) (ha_partition.cc:6542)
==11035== by 0x7F4CB0: compare_tables(st_table*, Alter_info*, st_ha_create_information*, unsigned int, enum_alter_table_change_level*, st_key**, unsigned int**, unsigned int*, unsigned int**, unsigned int*, unsigned int*) (sql_table.cc:6065)
==11035== by 0x7F78C1: mysql_alter_table(THD*, char*, char*, st_ha_create_information*, TABLE_LIST*, Alter_info*, unsigned int, st_order*, bool) (sql_table.cc:7098)
==11035== by 0x68C41C: mysql_execute_command(THD*) (sql_parse.cc:3069)
==11035== by 0x696008: mysql_parse(THD*, char*, unsigned int, char const**) (sql_parse.cc:6217)
==11035== by 0x687E9C: dispatch_command(enum_server_command, THD*, char*, unsigned int) (sql_parse.cc:1294)
==11035== by 0x686D97: do_command(THD*) (sql_parse.cc:906)
==11035== by 0x683D30: handle_one_connection (sql_connect.cc:1208)
==11035== by 0x54F7EFB: start_thread (pthread_create.c:304)
==11035== by 0x57F059C: clone (clone.S:112)

Reproducible on MariaDB 5.1 revno 3151, MariaDB 5.2, 5.3, 5.5.
Two statements above is the complete test case.