psbt: handle 'unsetting' final witness stack

Prior to this commit, passing a NULL stack to `bitcoin_tx_input_set_witness`
unsets the witness stack on the bitcoin_tx's wally_tx but leaves the
final witness on the PSBT unchanged.

at the moment, libwally's `wally_psbt_input_set_final_witness` will blow
up if you attempt to set a NULL witness -- instead we manually remove it
if the passed in stack is NULL. previously we would leave the PSBT's
witness unchanged.

bitcoin/tx.c

@@ -309,7 +309,6 @@ void bitcoin_tx_input_set_witness(struct bitcoin_tx *tx, int innum,
 {
 	struct wally_tx_witness_stack *stack = NULL;
 	size_t stack_size = tal_count(witness);
-	struct wally_psbt_input *in;
 
 	/* Free any lingering witness */
 	if (witness) {
@@ -321,17 +320,21 @@ void bitcoin_tx_input_set_witness(struct bitcoin_tx *tx, int innum,
 	wally_tx_set_input_witness(tx->wtx, innum, stack);
 
 	/* Also add to the psbt */
-	if (stack) {
-		assert(innum < tx->psbt->num_inputs);
-		in = &tx->psbt->inputs[innum];
-		wally_psbt_input_set_final_witness(in, stack);
+	if (stack)
+		wally_psbt_input_set_final_witness(&tx->psbt->inputs[innum], stack);
+	else {
+		/* FIXME: libwally-psbt doesn't allow 'unsetting' of witness via
+		 * the set method at the moment, so we do it manually*/
+		struct wally_psbt_input *in = &tx->psbt->inputs[innum];
+		if (in->final_witness)
+			wally_tx_witness_stack_free(in->final_witness);
+		in->final_witness = NULL;
 	}
 
 	if (stack)
 		wally_tx_witness_stack_free(stack);
 	if (taken(witness))
 	    tal_free(witness);
-
 }
 
 void bitcoin_tx_input_set_script(struct bitcoin_tx *tx, int innum, u8 *script)