rdma: bugfix: ram_control_save_page()

We were not checking for a valid 'bytes_sent' pointer before accessing it.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael R. Hines <mrhines@us.ibm.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
