include/net/genetlink.h: Allow genlmsg_cancel to accept a NULL argument

nlmsg_cancel can accept NULL as its second argument, so for similarity,
this patch extends genlmsg_cancel to be able to accept a NULL second
argument as well.
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/genetlink.h

@@ -195,6 +195,7 @@ static inline int genlmsg_end(struct sk_buff *skb, void *hdr)
  */
 static inline void genlmsg_cancel(struct sk_buff *skb, void *hdr)
 {
+	if (hdr)
 		nlmsg_cancel(skb, hdr - GENL_HDRLEN - NLMSG_HDRLEN);
 }