fix GetScriptForMining() CReserveKey::keepKey() issue

src/wallet/wallet.cpp

@@ -2586,12 +2586,11 @@ void CWallet::UpdatedTransaction(const uint256 &hashTx)
 void CWallet::GetScriptForMining(CScript &script)
 {
     CReserveKey reservekey(this);
-    reservekey.KeepKey();
-    
     CPubKey pubkey;
     if (!reservekey.GetReservedKey(pubkey))
         return;
     script = CScript() << ToByteVector(pubkey) << OP_CHECKSIG;
+    reservekey.KeepKey();
 }
 
 void CWallet::LockCoin(COutPoint& output)