Bugfixes walletclass

Some problems found by ius:
* compiler complains with no return after critical section block
* CKeyStore::GetPrivKey(key) was undefined for unknown key
* missing return statement in GetChange()

src/wallet.h

@@ -96,7 +96,7 @@ public:
     {
         if (!MoneyRange(txout.nValue))
             throw std::runtime_error("CWallet::GetChange() : value out of range");
-        if (IsChange(txout) ? txout.nValue : 0);
+        return (IsChange(txout) ? txout.nValue : 0);
     }
     bool IsMine(const CTransaction& tx) const
     {