Clean up change computation in CreateTransaction.

Compute the change directly as difference between the "requested" and
the actual value returned by SelectCoins.  This removes a duplication of
the fee logic code.

src/wallet/wallet.cpp

@@ -1757,9 +1757,9 @@ bool CWallet::CreateTransaction(const vector<CRecipient>& vecSend,
                 nChangePosRet = -1;
                 bool fFirst = true;
 
-                CAmount nTotalValue = nValue;
+                CAmount nValueToSelect = nValue;
                 if (nSubtractFeeFromAmount == 0)
-                    nTotalValue += nFeeRet;
+                    nValueToSelect += nFeeRet;
                 double dPriority = 0;
                 // vouts to the payees
                 BOOST_FOREACH (const CRecipient& recipient, vecSend)
@@ -1796,7 +1796,7 @@ bool CWallet::CreateTransaction(const vector<CRecipient>& vecSend,
                 // Choose coins to use
                 set<pair<const CWalletTx*,unsigned int> > setCoins;
                 CAmount nValueIn = 0;
-                if (!SelectCoins(nTotalValue, setCoins, nValueIn, coinControl))
+                if (!SelectCoins(nValueToSelect, setCoins, nValueIn, coinControl))
                 {
                     strFailReason = _("Insufficient funds");
                     return false;
@@ -1814,10 +1814,7 @@ bool CWallet::CreateTransaction(const vector<CRecipient>& vecSend,
                     dPriority += (double)nCredit * age;
                 }
 
-                CAmount nChange = nValueIn - nValue;
-                if (nSubtractFeeFromAmount == 0)
-                    nChange -= nFeeRet;
-
+                const CAmount nChange = nValueIn - nValueToSelect;
                 if (nChange > 0)
                 {
                     // Fill a vout to ourself