Fix crash sending protocols without a greeting.

adsbus/send.c

@@ -138,8 +138,10 @@ void send_new_wrapper(int fd, void *passthrough, struct peer *on_close) {
 
 void send_hello(struct buf **buf_pp, void *passthrough) {
 	struct serializer *serializer = (struct serializer *) passthrough;
+	if (serializer->hello) {
 		serializer->hello(buf_pp);
 	}
+}
 
 void send_write(struct packet *packet) {
 	packet_sanity_check(packet);