elements: Fix transaction size estimate when selecting coins

In elements we add an explicit fee output, if we don't consider it when
selecting coins, we end up underpaying the fees.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

wallet/wallet.c

@@ -378,9 +378,13 @@ static const struct utxo **wallet_select(const tal_t *ctx, struct wallet *w,
 		weight += 2 * 4;
 
 		/* Each output additionally has an asset_tag (1 + 32), value
-		 * is prefixed by a version (1 byte) and an empty nonce (1
-		 * byte). */
+		 * is prefixed by a version (1 byte), an empty nonce (1
+		 * byte), two empty proofs (2 bytes). */
 		weight += (32 + 1 + 1 + 1) * 4 * num_outputs;
+
+		/* An elements transaction has 1 additional output for fees */
+		weight += (8 + 1) * 4; /* Bitcoin style output */
+		weight += (32 + 1 + 1 + 1) * 4; /* Elements added fields */
 	}
 
 	*fee_estimate = AMOUNT_SAT(0);
@@ -420,6 +424,10 @@ static const struct utxo **wallet_select(const tal_t *ctx, struct wallet *w,
 		/* Account for witness (1 byte count + sig + key) */
 		input_weight += 1 + (1 + 73 + 1 + 33);
 
+		/* Elements inputs have 6 bytes of blank proofs attached. */
+		if (is_elements)
+			input_weight += 6;
+
 		weight += input_weight;
 
 		if (!amount_sat_add(satoshi_in, *satoshi_in, u->amount))