net: avoid NULL deref in napi_get_frags()

napi_alloc_skb() can return NULL.
We should not crash should this happen.

Fixes: 93f93a440415 ("net: move skb_mark_napi_id() into core networking stack")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -4390,8 +4390,10 @@ struct sk_buff *napi_get_frags(struct napi_struct *napi)
 
 	if (!skb) {
 		skb = napi_alloc_skb(napi, GRO_MAX_HEAD);
-		napi->skb = skb;
-		skb_mark_napi_id(skb, napi);
+		if (skb) {
+			napi->skb = skb;
+			skb_mark_napi_id(skb, napi);
+		}
 	}
 	return skb;
 }