bitcoin: avoid adding to NULL

Detected by UBSan:

$ UBSAN_OPTIONS=print_stacktrace=1 ./wallet/test/run-psbt_fixup

bitcoin/psbt.c:733:2: runtime error: applying zero offset to null pointer
    #0 0x53c829 in psbt_from_bytes lightning/bitcoin/psbt.c:733:2
    #1 0x5adcb0 in main lightning/wallet/test/run-psbt_fixup.c:174:10

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior bitcoin/psbt.c:733:2

Author: morehouse

bitcoin/psbt.c

@@ -656,12 +656,14 @@ struct wally_tx *psbt_final_tx(const tal_t *ctx, const struct wally_psbt *psbt)
 		return NULL;
 
 	tal_wally_start();
-	if (wally_psbt_extract(psbt, /* flags */ 0, &wtx) == WALLY_OK)
+	if (wally_psbt_extract(psbt, /* flags */ 0, &wtx) == WALLY_OK) {
 		tal_add_destructor(wtx, wally_tx_destroy);
-	else
+		tal_wally_end_onto(ctx, wtx, struct wally_tx);
+	} else {
 		wtx = NULL;
+		tal_wally_end(ctx);
+	}
 
-	tal_wally_end_onto(ctx, wtx, struct wally_tx);
 	return wtx;
 }
 
@@ -673,11 +675,13 @@ struct wally_psbt *psbt_from_b64(const tal_t *ctx,
 	char *str = tal_strndup(tmpctx, b64, b64len);
 
 	tal_wally_start();
-	if (wally_psbt_from_base64(str, /* flags */ 0, &psbt) == WALLY_OK)
+	if (wally_psbt_from_base64(str, /* flags */ 0, &psbt) == WALLY_OK) {
 		tal_add_destructor(psbt, psbt_destroy);
-	else
+		tal_wally_end_onto(ctx, psbt, struct wally_psbt);
+	} else {
 		psbt = NULL;
-	tal_wally_end_onto(ctx, psbt, struct wally_psbt);
+		tal_wally_end(ctx);
+	}
 
 	return psbt;
 }
@@ -726,11 +730,13 @@ struct wally_psbt *psbt_from_bytes(const tal_t *ctx, const u8 *bytes,
 	struct wally_psbt *psbt;
 
 	tal_wally_start();
-	if (wally_psbt_from_bytes(bytes, byte_len, /* flags */ 0, &psbt) == WALLY_OK)
+	if (wally_psbt_from_bytes(bytes, byte_len, /* flags */ 0, &psbt) == WALLY_OK) {
 		tal_add_destructor(psbt, psbt_destroy);
-	else
+		tal_wally_end_onto(ctx, psbt, struct wally_psbt);
+	} else {
 		psbt = NULL;
-	tal_wally_end_onto(ctx, psbt, struct wally_psbt);
+		tal_wally_end(ctx);
+	}
 
 	return psbt;
 }