wallet: CommitTransaction, remove extra wtx lookup and add exception for a possible db write error.

1) `Wallet::AddToWallet` is already returning the pointer to the inserted `CWalletTx`, so there is no need to look it up in the map again.

2) `Wallet::AddToWallet` can only return a nullptr if the db `writeTx` call failed. Which should be treated as an error.

Author: furszy

src/wallet/wallet.cpp

@@ -2106,7 +2106,7 @@ void CWallet::CommitTransaction(CTransactionRef tx, mapValue_t mapValue, std::ve
 
     // Add tx to wallet, because if it has change it's also ours,
     // otherwise just for transaction history.
-    AddToWallet(tx, TxStateInactive{}, [&](CWalletTx& wtx, bool new_tx) {
+    CWalletTx* wtx = AddToWallet(tx, TxStateInactive{}, [&](CWalletTx& wtx, bool new_tx) {
         CHECK_NONFATAL(wtx.mapValue.empty());
         CHECK_NONFATAL(wtx.vOrderForm.empty());
         wtx.mapValue = std::move(mapValue);
@@ -2116,24 +2116,25 @@ void CWallet::CommitTransaction(CTransactionRef tx, mapValue_t mapValue, std::ve
         return true;
     });
 
+    // wtx can only be null if the db write failed.
+    if (!wtx) {
+        throw std::runtime_error(std::string(__func__) + ": Wallet db error, transaction commit failed");
+    }
+
     // Notify that old coins are spent
     for (const CTxIn& txin : tx->vin) {
         CWalletTx &coin = mapWallet.at(txin.prevout.hash);
         coin.MarkDirty();
         NotifyTransactionChanged(coin.GetHash(), CT_UPDATED);
     }
 
-    // Get the inserted-CWalletTx from mapWallet so that the
-    // wtx cached mempool state is updated correctly
-    CWalletTx& wtx = mapWallet.at(tx->GetHash());
-
     if (!fBroadcastTransactions) {
         // Don't submit tx to the mempool
         return;
     }
 
     std::string err_string;
-    if (!SubmitTxMemoryPoolAndRelay(wtx, err_string, true)) {
+    if (!SubmitTxMemoryPoolAndRelay(*wtx, err_string, true)) {
         WalletLogPrintf("CommitTransaction(): Transaction cannot be broadcast immediately, %s\n", err_string);
         // TODO: if we expect the failure to be long term or permanent, instead delete wtx from the wallet and return failure.
     }