Treat a path with AS 0 anywhere as a withdrawal (closes #141)

An AS path that holds AS 0 anywhere, a member of an AS set included, now
decodes as an empty path. RFC 7607 makes such a path malformed and its
announcements withdrawals. An empty path already gets that handling:
the prefix handler deletes the peer's route for the prefix, and the AS
and peering handlers record nothing from it. Before, a path such as
[64496, 0, 64497] stored the route with origin AS64497, recorded both
ASes, and recorded the peerings between the path's other ASes.

Model: opus-5-5

internal/routewatch/prefixhandler_internal_test.go

@@ -381,11 +381,12 @@ func TestPrefixHandlerDoesNotStoreAnAnnouncementWithAnEmptyPath(t *testing.T) {
 // TestPrefixHandlerRemovesTheRouteOnAnAnnouncementWithoutAnOrigin runs, through
 // the prefix handler into a real database, a peer's announcement of a prefix
 // with origin AS testASNB and then the peer's announcement of the prefix with
-// an AS path that gives no origin: one whose last AS is 0, or an empty one. As
-// any later announcement from the peer does, the second replaces the peer's
-// route, here with none, whether it comes in a later batch or follows a
-// withdrawal in one batch. The in-memory route count and prefix distribution
-// must match the route table.
+// an AS path that gives no origin: one whose last AS is 0, an empty one, or one
+// that holds AS 0 before its end, which decodes as an empty path. As any later
+// announcement from the peer does, the second replaces the peer's route, here
+// with none, whether it comes in a later batch or follows a withdrawal in one
+// batch. The in-memory route count and prefix distribution must match the
+// route table.
 func TestPrefixHandlerRemovesTheRouteOnAnAnnouncementWithoutAnOrigin(
 	t *testing.T,
 ) {
@@ -421,6 +422,12 @@ func TestPrefixHandlerRemovesTheRouteOnAnAnnouncementWithoutAnOrigin(
 			name:  "an empty path",
 			batch: []*ristypes.RISMessage{noOrigin(ristypes.ASPath{})},
 		},
+		{
+			name: "a path with AS 0 before its end",
+			batch: []*ristypes.RISMessage{
+				noOrigin(decodePath(t, testASNA, 0, testASNB)),
+			},
+		},
 		{
 			name:  "a withdrawal and then a path whose last AS is 0",
 			batch: []*ristypes.RISMessage{withdrawal, noOrigin(endsInZero)},