gro: Fix error handling on extremely short frags

When a frag is shorter than an Ethernet header, we'd return a
zeroed packet instead of aborting.  This patch fixes that.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2536,6 +2536,7 @@ struct sk_buff *napi_fraginfo_skb(struct napi_struct *napi,
 
 	if (!pskb_may_pull(skb, ETH_HLEN)) {
 		napi_reuse_skb(napi, skb);
+		skb = NULL;
 		goto out;
 	}