invoice: remove strange code.

This is called when we load from database: clearly our tests aren't thorough
enough because we were allocating and initializing `r` in an unused structure.

invs is also the owner already; functions which steal are a bit surprising
to callers, so we either document them, or just don't do it.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/invoice.c

@@ -60,9 +60,7 @@ static struct invoice *find_invoice_by_label(const struct invoices *invs,
 void invoice_add(struct invoices *invs,
 		 struct invoice *inv)
 {
-	tal_steal(invs, inv);
-	struct invoice *invoice = tal(invs, struct invoice);
-	sha256(&invoice->rhash, invoice->r.r, sizeof(invoice->r.r));
+	sha256(&inv->rhash, inv->r.r, sizeof(inv->r.r));
 	list_add(&invs->invlist, &inv->list);
 }