net: fix potential null pointer dereference

Add null check to avoid a potential null pointer dereference.

Addresses-Coverity-ID: 1408831
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>