Mroonga has memory leak in ha_mroonga::is_foreign_key_field

mtr --valgrind mroonga/storage.update_virtual_column
==6093== 40 bytes in 1 blocks are definitely lost in loss record 2 of 12
==6093== at 0x4C2E2BB: malloc (vg_replace_malloc.c:309)
==6093== by 0xF759E47: grn_malloc_default (alloc.c:780)
==6093== by 0xF434920: accessor_new (db.c:5543)
==6093== by 0xF435091: grn_obj_get_accessor (db.c:5748)
==6093== by 0xF42DCC7: grn_obj_column (db.c:4682)
==6093== by 0xF3B0518: ha_mroonga::is_foreign_key_field(char const*, char const*) (ha_mroonga.cpp:8979)
==6093== by 0xF3A3392: ha_mroonga::storage_update_row(unsigned char const*, unsigned char const*) (ha_mroonga.cpp:6661)
==6093== by 0xF3A52F7: ha_mroonga::update_row(unsigned char const*, unsigned char const*) (ha_mroonga.cpp:7021)
==6093== by 0xBB7C56: handler::ha_update_row(unsigned char const*, unsigned char const*) (handler.cc:6680)
==6093== by 0x99F18C: mysql_update(THD*, TABLE_LIST*, List<Item>&, List<Item>&, Item*, unsigned int, st_order*, unsigned long long, bool, unsigned long long*, unsigned long long*) (sql_update.cc:1045)
==6093== by 0x88EEDB: mysql_execute_command(THD*) (sql_parse.cc:4358)
==6093== by 0x89AFCE: mysql_parse(THD*, char*, unsigned int, Parser_state*, bool, bool) (sql_parse.cc:7943)
==6093== by 0x887C17: dispatch_command(enum_server_command, THD*, char*, unsigned int, bool, bool) (sql_parse.cc:1853)
==6093== by 0x886489: do_command(THD*) (sql_parse.cc:1369)
==6093== by 0xA101D5: do_handle_one_connection(CONNECT*, bool) (sql_connect.cc:1414)
==6093== by 0xA0FF09: handle_one_connection (sql_connect.cc:1309)
==6093== by 0xE41174: pfs_spawn_thread (pfs.cc:1862)
==6093== by 0x4E42568: start_thread (in /lib64/libpthread-2.26.so)
==6093== by 0x6ECBA2E: clone (in /lib64/libc-2.26.so)