[UDP]: Reread uh pointer after pskb_trim

The header may have moved when trimming.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1215,6 +1215,7 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct hlist_head udptable[],
 
 		if (ulen < sizeof(*uh) || pskb_trim_rcsum(skb, ulen))
 			goto short_packet;
+		uh = skb->h.uh;
 
 		udp4_csum_init(skb, uh);