fixup finish_psbt change creation for elements

Author: instagibbs

wallet/reservation.c

@@ -367,6 +367,7 @@ static struct command_result *finish_psbt(struct command *cmd,
 	/* Should we add a change output for the excess? */
 	if (excess_as_change) {
 		struct amount_sat change;
+		struct pubkey pubkey;
 		s64 keyidx;
 		u8 *b32script;
 
@@ -384,7 +385,12 @@ static struct command_result *finish_psbt(struct command *cmd,
 					    "Failed to generate change address."
 					    " Keys exhausted.");
 
-		b32script = p2tr_for_keyidx(tmpctx, cmd->ld, keyidx);
+		if (chainparams->is_elements) {
+			bip32_pubkey(cmd->ld, &pubkey, keyidx);
+			b32script = scriptpubkey_p2wpkh(tmpctx, &pubkey);
+		} else {
+			b32script = p2tr_for_keyidx(tmpctx, cmd->ld, keyidx);
+		}
 		if (!b32script) {
 			return command_fail(cmd, LIGHTNINGD,
 					    "Failed to generate change address."