removed unnecessary calculation of cache value 
 otherwise Join::preapre tries to calculate 
 undefined values(filed values)


mysql-test/r/type_decimal.result:
  Fix for bug#17826 'type_decimal' fails with ps-protocol
   test case, this test case reproduce the same bug but without PS protocol
mysql-test/t/type_decimal.test:
  Fix for bug#17826 'type_decimal' fails with ps-protocol
   test case, this test case reproduce the same bug but without PS protocol