NET: ROSE: Don't dereference NULL neighbour pointer.

A ROSE socket doesn't necessarily always have a neighbour pointer so check
if the neighbour pointer is valid before dereferencing it.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Tested-by: Bernard Pidoux <f6bvp@free.fr>
Cc: stable@vger.kernel.org #2.6.11+
Signed-off-by: David S. Miller <davem@davemloft.net>