tipc: fix a possible memleak in tipc_buf_append

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: b7df21cf1b79 ("tipc: skb_linearize the head skb when reassembling msgs")
Reported-by: Paolo Abeni <[email protected]>
Signed-off-by: Xin Long <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Reviewed-by: Tung Nguyen <[email protected]>
Link: https://lore.kernel.org/r/90710748c29a1521efac4f75ea01b3b7e61414cf.1714485818.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <[email protected]>