Fix parsing of compressed IPv4 Updates

We were using the Flags field instead of the AE field when parsing
compressed IPv4 prefixes in Update messages.  This bug shouldn't
trigger, since we never use compression with IPv4.

message.c

@@ -348,7 +348,7 @@ parse_packet(const unsigned char *from, struct interface *ifp,
             DO_NTOHS(seqno, message + 8);
             DO_NTOHS(metric, message + 10);
             if(message[5] == 0 ||
-               (message[3] == 1 ? have_v4_prefix : have_v6_prefix))
+               (message[2] == 1 ? have_v4_prefix : have_v6_prefix))
                 rc = network_prefix(message[2], message[4], message[5],
                                     message + 12,
                                     message[2] == 1 ? v4_prefix : v6_prefix,