If none of the slave interfaces are specified, struct nlattr *data[] may
be NULL. Make sure to check for that.

While I'm at it, fix the horrible error messages displayed when only one
of the slave interfaces isn't specified.
Signed-off-by: Arvid Brodin <arvid.brodin@alten.se>
Signed-off-by: David S. Miller <davem@davemloft.net>