Refuse to pathfind when provided our_node_id matches internal dummy pk

The fuzzer managed to hit this and it causes some invalid paths to be generated
internally.

Author: valentinewallace

lightning/src/routing/router.rs

@@ -1489,6 +1489,9 @@ where L::Target: Logger {
 	if payee_node_id_opt.map_or(false, |payee| payee == our_node_id) {
 		return Err(LightningError{err: "Cannot generate a route to ourselves".to_owned(), action: ErrorAction::IgnoreError});
 	}
+	if our_node_id == maybe_dummy_payee_node_id {
+		return Err(LightningError{err: "Invalid origin node id provided, use a different one".to_owned(), action: ErrorAction::IgnoreError});
+	}
 
 	if final_value_msat > MAX_VALUE_MSAT {
 		return Err(LightningError{err: "Cannot generate a route of more value than all existing satoshis".to_owned(), action: ErrorAction::IgnoreError});