fix prevtx hashing when input count is 0

firmware/signing.c

@@ -334,7 +334,12 @@ void signing_txack(TransactionType *tx)
 			tx_init(&tp, tx->inputs_cnt, tx->outputs_cnt, tx->version, tx->lock_time, tx->extra_data_len, false);
 			progress_meta_step = progress_step / (tp.inputs_len + tp.outputs_len);
 			idx2 = 0;
-			send_req_2_prev_input();
+			if (tp.inputs_len > 0) {
+				send_req_2_prev_input();
+			} else {
+				tx_serialize_header_hash(&tp);
+				send_req_2_prev_output();
+			}
 			return;
 		case STAGE_REQUEST_2_PREV_INPUT:
 			progress = (idx1 * progress_step + idx2 * progress_meta_step) >> PROGRESS_PRECISION;

firmware/transaction.h

@@ -55,6 +55,7 @@ uint32_t tx_serialize_input(TxStruct *tx, const TxInputType *input, uint8_t *out
 uint32_t tx_serialize_output(TxStruct *tx, const TxOutputBinType *output, uint8_t *out);
 
 void tx_init(TxStruct *tx, uint32_t inputs_len, uint32_t outputs_len, uint32_t version, uint32_t lock_time, uint32_t extra_data_len, bool add_hash_type);
+uint32_t tx_serialize_header_hash(TxStruct *tx);
 uint32_t tx_serialize_input_hash(TxStruct *tx, const TxInputType *input);
 uint32_t tx_serialize_output_hash(TxStruct *tx, const TxOutputBinType *output);
 uint32_t tx_serialize_extra_data_hash(TxStruct *tx, const uint8_t *data, uint32_t datalen);