[UDP]: Reread uh pointer after pskb_trim

The header may have moved when trimming.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index ce6c460..fc620a7 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1215,6 +1215,7 @@
 
 		if (ulen < sizeof(*uh) || pskb_trim_rcsum(skb, ulen))
 			goto short_packet;
+		uh = skb->h.uh;
 
 		udp4_csum_init(skb, uh);