[IPV4]: Prevent oops when printing martian source

In some cases, we may be generating packets with a source address that
qualifies as martian. This can happen when we're in the middle of setting
up the network, and netfilter decides to reject a packet with an RST.
The IPv4 routing code would try to print a warning and oops, because
locally generated packets do not have a valid skb->mac.raw pointer
at this point.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1685,7 +1685,7 @@ static void ip_handle_martian_source(struct net_device *dev,
 		printk(KERN_WARNING "martian source %u.%u.%u.%u from "
 			"%u.%u.%u.%u, on dev %s\n",
 			NIPQUAD(daddr), NIPQUAD(saddr), dev->name);
-		if (dev->hard_header_len) {
+		if (dev->hard_header_len && skb->mac.raw) {
 			int i;
 			unsigned char *p = skb->mac.raw;
 			printk(KERN_WARNING "ll header: ");