[NETLINK]: Fix RTA_NEST_CANCEL().

Only skb_trim() if 'start' is non-NULL.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/rtnetlink.h

@@ -924,7 +924,8 @@ extern void __rta_fill(struct sk_buff *skb, int attrtype, int attrlen, const voi
 	(skb)->len; })
 
 #define RTA_NEST_CANCEL(skb, start) \
-({	skb_trim(skb, (unsigned char *) (start) - (skb)->data); \
+({	if (start) \
+		skb_trim(skb, (unsigned char *) (start) - (skb)->data); \
 	-1; })
 
 #define RTA_GET_U32(rta) \