vpn: ignore unexpected default-route for nm_vpn_connection_ip6_config_get()

The VPN data comes from an external source, it may be bogus.
Default-routes are not allowed on this point and would trigger
an assertion afterwards. Skip over them.

(cherry picked from commit 071103b172)

src/vpn-manager/nm-vpn-connection.c

@@ -1640,7 +1640,7 @@ nm_vpn_connection_ip6_config_get (NMVpnConnection *self, GVariant *dict)
 			if (!ip6_addr_from_variant (dest, &route.network))
 				goto next;
 
-			if (prefix > 128)
+			if (prefix > 128 || prefix == 0)
 				goto next;
 
 			route.plen = prefix;