Fixed heap_btree test failure on 64-bit boxes.

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CREATE TABLE t1(val INT, KEY USING BTREE(val)) ENGINE=memory;

INSERT INTO t1 VALUES(0);

--replace_result 37 21

SELECT INDEX_LENGTH FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA=DATABASE() AND TABLE_NAME='t1';

UPDATE t1 SET val=1;

--replace_result 37 21

SELECT INDEX_LENGTH FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA=DATABASE() AND TABLE_NAME='t1';

DROP TABLE t1;