elements: Do not get upset if we see a confidential asset or value

I made some assumptions that turn out not to be true, mea culpa.

bitcoin/tx.c

@@ -204,6 +204,8 @@ const u8 *bitcoin_tx_output_get_script(const tal_t *ctx,
 	return res;
 }
 
+/* FIXME(cdecker) Make the caller pass in a reference to amount_asset, and
+ * return false if unintelligible/encrypted. (WARN UNUSED). */
 struct amount_asset bitcoin_tx_output_get_amount(const struct bitcoin_tx *tx,
 						 int outnum)
 {
@@ -216,13 +218,18 @@ struct amount_asset bitcoin_tx_output_get_amount(const struct bitcoin_tx *tx,
 	output = &tx->wtx->outputs[outnum];
 
 	if (chainparams->is_elements) {
-		/* We currently only support v1 asset tags */
-		assert(output->asset_len == sizeof(amount.asset) &&
-		       output->asset[0] == 0x01);
+		assert(output->asset_len == sizeof(amount.asset));
 		memcpy(&amount.asset, output->asset, sizeof(amount.asset));
+
+		/* We currently only support explicit value asset tags, others
+		 * are confidential, so don't even try to assign a value to
+		 * it. */
+		if (output->asset[0] == 0x01) {
 			memcpy(&raw, output->value + 1, sizeof(raw));
 			amount.value = be64_to_cpu(raw);
-
+		} else {
+			amount.value = 0;
+		}
 	} else {
 		/* Do not assign amount.asset, we should never touch it in
 		 * non-elements scenarios. */