permute_tx: bail on empty permute_{inputs/outputs} arguments

permute_outputs is sometimes called with empty arguments from initial_commit_tx.
Make sure we guard against that case. We also do the same in permute_inputs for
good measure.

Signed-off-by: William Casarin <jb55@jb55.com>

common/permute_tx.c

@@ -59,6 +59,10 @@ void permute_inputs(struct bitcoin_tx_input *inputs, size_t num_inputs,
 {
 	size_t i;
 
+	/* We can't permute nothing! */
+	if (num_inputs == 0)
+		return;
+
 	/* Now do a dumb sort (num_inputs is small). */
 	for (i = 0; i < num_inputs-1; i++) {
 		/* Swap best into first place. */
@@ -126,6 +130,10 @@ void permute_outputs(struct bitcoin_tx_output *outputs, size_t num_outputs,
 {
 	size_t i;
 
+	/* We can't permute nothing! */
+	if (num_outputs == 0)
+		return;
+
 	/* Now do a dumb sort (num_outputs is small). */
 	for (i = 0; i < num_outputs-1; i++) {
 		/* Swap best into first place. */