drbd: Ensure that data_size is not 0 before using data_size-1 as index

This could be exploited by a peer which runs modified code.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>