Fix potential mod by 0 in DecimalValue.

This can happen in the case where there is also overflow, in which case
we might take the mod of an uninitialized value. If that value is 0, we
trap.

Change-Id: I0f455de8405512e841d518330ff1e985b225b954
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5392
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins