[IPV6]: Fix oops in rt6_device_match()

This fixes panic in rt6_device_match().

Well, rt->rt6i_idev is always set if it is dynamically allocated.
However, when we hit ip6_null_entry here, its rt6i_idev is NULL.
This patch is minimum fix to avoid the oops for now.
Signed-off-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -184,7 +184,8 @@ static __inline__ struct rt6_info *rt6_device_match(struct rt6_info *rt,
 			if (dev->ifindex == oif)
 				return sprt;
 			if (dev->flags & IFF_LOOPBACK) {
-				if (sprt->rt6i_idev->dev->ifindex != oif) {
+				if (sprt->rt6i_idev == NULL ||
+				    sprt->rt6i_idev->dev->ifindex != oif) {
 					if (strict && oif)
 						continue;
 					if (local && (!oif ||