neigh: check error pointer instead of NULL for ipv4_neigh_lookup()

Fixes: commit f187bc6efb7250afee0e2009b6106 ("ipv4: No need to set generic neighbour pointer")
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 173e7ea..cbadb94 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -746,7 +746,7 @@
 	}
 
 	n = ipv4_neigh_lookup(&rt->dst, NULL, &new_gw);
-	if (n) {
+	if (!IS_ERR(n)) {
 		if (!(n->nud_state & NUD_VALID)) {
 			neigh_event_send(n, NULL);
 		} else {