Altering decimal column on a TokuDB table can corrupt data

When changing a decimal column's scale but not precision on a tokudb column (and keeping everything else the same) the column's data can be corrupted.

NB Changing NULL/NOT NULL, SIGNED/UNSIGNED or decimal precision will all result in things working properly.

Here's a test case that can reproduce the problem:

create table dec_test( 

	val decimal(9,2) NOT NULL

) ENGINE = TOKUDB;

INSERT INTO `dec_test` VALUES (0);

INSERT INTO `dec_test` VALUES (10);

INSERT INTO `dec_test` VALUES (20);

ALTER TABLE `dec_test` CHANGE COLUMN `val` `val` DECIMAL(9,4) NOT NULL; 

select  * from dec_test;