walletAPI: correct confirmations in txHistory for unsynced wallets

src/wallet/api/transaction_history.cpp

@@ -134,7 +134,7 @@ void TransactionHistoryImpl::refresh()
         ti->m_subaddrAccount = pd.m_subaddr_index.major;
         ti->m_label     = m_wallet->m_wallet->get_subaddress_label(pd.m_subaddr_index);
         ti->m_timestamp = pd.m_timestamp;
-        ti->m_confirmations = wallet_height - pd.m_block_height;
+        ti->m_confirmations = (wallet_height > pd.m_block_height) ? wallet_height - pd.m_block_height : 0;
         ti->m_unlock_time = pd.m_unlock_time;
         m_history.push_back(ti);
 
@@ -177,7 +177,7 @@ void TransactionHistoryImpl::refresh()
         ti->m_subaddrAccount = pd.m_subaddr_account;
         ti->m_label = pd.m_subaddr_indices.size() == 1 ? m_wallet->m_wallet->get_subaddress_label({pd.m_subaddr_account, *pd.m_subaddr_indices.begin()}) : "";
         ti->m_timestamp = pd.m_timestamp;
-        ti->m_confirmations = wallet_height - pd.m_block_height;
+        ti->m_confirmations = (wallet_height > pd.m_block_height) ? wallet_height - pd.m_block_height : 0;
 
         // single output transaction might contain multiple transfers
         for (const auto &d: pd.m_dests) {