wallet: Have GetDebit use the wallet's TXO set

Instead of looking up the previous tx in mapWallet, and then calling
IsMine on the specified output, use the TXO set and its cached IsMine
value.

src/wallet/wallet.cpp

@@ -1543,16 +1543,10 @@ void CWallet::BlockUntilSyncedToCurrentChain() const {
 // and a not-"is mine" (according to the filter) input.
 CAmount CWallet::GetDebit(const CTxIn &txin, const isminefilter& filter) const
 {
-    {
-        LOCK(cs_wallet);
-        const auto mi = mapWallet.find(txin.prevout.hash);
-        if (mi != mapWallet.end())
-        {
-            const CWalletTx& prev = (*mi).second;
-            if (txin.prevout.n < prev.tx->vout.size())
-                if (IsMine(prev.tx->vout[txin.prevout.n]) & filter)
-                    return prev.tx->vout[txin.prevout.n].nValue;
-        }
+    LOCK(cs_wallet);
+    auto txo = GetTXO(txin.prevout);
+    if (txo && (txo->GetIsMine() & filter)) {
+        return txo->GetTxOut().nValue;
     }
     return 0;
 }