tipc: fix a possible memleak in tipc_buf_append

__skb_linearize() doesn't free the skb when it fails, so move
'*buf = NULL' after __skb_linearize(), so that the skb can be
freed on the err path.

Fixes: b7df21cf ("tipc: skb_linearize the head skb when reassembling msgs")
Reported-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
Link: https://lore.kernel.org/r/90710748c29a1521efac4f75ea01b3b7e61414cf.1714485818.git.lucien.xin@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/tipc/msg.c

@@ -142,9 +142,9 @@ int tipc_buf_append(struct sk_buff **headbuf, struct sk_buff **buf)
 	if (fragid == FIRST_FRAGMENT) {
 		if (unlikely(head))
 			goto err;
-		*buf = NULL;
 		if (skb_has_frag_list(frag) && __skb_linearize(frag))
 			goto err;
+		*buf = NULL;
 		frag = skb_unshare(frag, GFP_ATOMIC);
 		if (unlikely(!frag))
 			goto err;