peer: make id a pointer, NULL until we know peer's ID.

Much better than undefined, and testing for NULL is better than
testing for STATE_INIT.

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

daemon/peer.h

@@ -137,7 +137,7 @@ struct peer {
 	struct netaddr addr;
 
 	/* Their ID. */
-	struct pubkey id;
+	struct pubkey *id;
 
 	/* Current received packet. */
 	Pkt *inpkt;