htlc_wire: be stricter in marshaling/unmarshaling struct failed_htlc.

There are three cases:
1. failcode is 0, scid is NULL, failreason is the onion to fwd.
2. failcode is non-zero, but UPDATE bit not set.  scid is NULL, failreason NULL.
3. failcode has UPDATE bit set.  scid is non-NULL, failreason is NULL.

Assert these on marshaling, and only send the parts we need so unmarshal is
always canonical.

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

common/htlc_wire.c

@@ -30,12 +30,18 @@ void towire_failed_htlc(u8 **pptr, const struct failed_htlc *failed)
 	assert(!failed->failcode || !tal_len(failed->failreason));
 	towire_u64(pptr, failed->id);
 	towire_u16(pptr, failed->failcode);
-	if (failed->failcode & UPDATE)
+	if (failed->failcode & UPDATE) {
+		assert(!failed->failreason);
 		towire_short_channel_id(pptr, failed->scid);
-	else
+	} else {
 		assert(!failed->scid);
-	towire_u16(pptr, tal_count(failed->failreason));
-	towire_u8_array(pptr, failed->failreason, tal_count(failed->failreason));
+		if (!failed->failcode) {
+			assert(failed->failreason);
+			towire_u16(pptr, tal_count(failed->failreason));
+			towire_u8_array(pptr, failed->failreason,
+					tal_count(failed->failreason));
+		}
+	}
 }
 
 void towire_htlc_state(u8 **pptr, const enum htlc_state hstate)
@@ -87,22 +93,26 @@ void fromwire_fulfilled_htlc(const u8 **cursor, size_t *max,
 
 struct failed_htlc *fromwire_failed_htlc(const tal_t *ctx, const u8 **cursor, size_t *max)
 {
-	u16 failreason_len;
 	struct failed_htlc *failed = tal(ctx, struct failed_htlc);
 
 	failed->id = fromwire_u64(cursor, max);
 	failed->failcode = fromwire_u16(cursor, max);
-	if (failed->failcode & UPDATE) {
-		failed->scid = tal(failed, struct short_channel_id);
-		fromwire_short_channel_id(cursor, max, failed->scid);
-	} else
+	if (failed->failcode == 0) {
+		u16 failreason_len;
 		failed->scid = NULL;
-	failreason_len = fromwire_u16(cursor, max);
-	if (failreason_len)
+		failreason_len = fromwire_u16(cursor, max);
 		failed->failreason = tal_arr(failed, u8, failreason_len);
-	else
+		fromwire_u8_array(cursor, max, failed->failreason,
+				  failreason_len);
+	} else {
 		failed->failreason = NULL;
-	fromwire_u8_array(cursor, max, failed->failreason, failreason_len);
+		if (failed->failcode & UPDATE) {
+			failed->scid = tal(failed, struct short_channel_id);
+			fromwire_short_channel_id(cursor, max, failed->scid);
+		} else {
+			failed->scid = NULL;
+		}
+	}
 
 	return failed;
 }