[NETROM]: Fix possible null pointer dereference.

If in nr_link_failed the neighbour list is non-empty but the node list
is empty we'll end dereferencing a  in a NULL pointer.

This fixes coverity 362.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>