Make GetAvailableCredit run GetHash() only once per transaction.

This makes the first getbalance/getinfo 63x faster on my wallet.

src/wallet.h

@@ -620,9 +620,10 @@ public:
             return nAvailableCreditCached;
 
         int64_t nCredit = 0;
+        uint256 hashTx = GetHash();
         for (unsigned int i = 0; i < vout.size(); i++)
         {
-            if (!pwallet->IsSpent(GetHash(), i))
+            if (!pwallet->IsSpent(hashTx, i))
             {
                 const CTxOut &txout = vout[i];
                 nCredit += pwallet->GetCredit(txout);