wireaddr: marshal empty address properly.

On unmarshal, we stop unmarshaling on a 0 (ADDR_TYPE_PADDING) type.  So
we should also stop marshaling in that case.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/wireaddr.c

@@ -28,6 +28,10 @@ bool fromwire_wireaddr(const u8 **cursor, size_t *max, struct wireaddr *addr)
 
 void towire_wireaddr(u8 **pptr, const struct wireaddr *addr)
 {
+	if (!addr || addr->type == ADDR_TYPE_PADDING) {
+		towire_u8(pptr, ADDR_TYPE_PADDING);
+		return;
+	}
 	towire_u8(pptr, addr->type);
 	towire(pptr, addr->addr, addr->addrlen);
 	towire_u16(pptr, addr->port);

common/wireaddr.h

@@ -36,6 +36,7 @@ struct wireaddr {
 	u16 port;
 };
 
+/* Inserts a single ADDR_TYPE_PADDING if addr is NULL */
 void towire_wireaddr(u8 **pptr, const struct wireaddr *addr);
 bool fromwire_wireaddr(const u8 **cursor, size_t *max, struct wireaddr *addr);
 #endif /* LIGHTNING_COMMON_WIREADDR_H */