notes: avoid potential use-after-free during insertion

The note_tree_insert() function may free the leaf_node struct we pass in (e.g., if it's a duplicate, or if it needs to be combined with an existing note). Most callers are happy with this, as they assume that ownership of the struct is handed off. But in load_subtree(), if we see an error we'll use the handed-off struct's key_oid to generate the die() message, potentially accessing freed memory. We can easily fix this by instead using the original oid that we copied into the leaf_node struct. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>