vm: fix unclaimedGas calculation

Fix difference with C#:
```
(base) ekaterinapavlova@MacBook-Air-4 neo-go % curl -X POST
http://seed1t5.neo.org:20332 -H 'Content-Type: application/json' -d '{
  "jsonrpc": "2.0",
  "method": "getapplicationlog",
  "params":
  ["61681ce24dffea5481e9a50b10159b43b7ebfc21967b0b06fee7ff69c7123e3f"],
  "id": 1
}' | json_pp
  % Total    % Received % Xferd  Average Speed   Time    Time     Time
  Current
                                 Dload  Upload   Total   Spent    Left
                                 Speed
100   429    0   269  100   160    582    346 --:--:-- --:--:-- --:--:--
   930
{
   "id" : 1,
   "jsonrpc" : "2.0",
   "result" : {
      "executions" : [
         {
            "exception" : null,
            "gasconsumed" : "198754",
            "notifications" : [],
            "stack" : [
               {
                  "type" : "Integer",
                  "value" : "0"
               }
            ],
            "trigger" : "Application",
            "vmstate" : "HALT"
         }
      ],
      "txid" :
      "0x61681ce24dffea5481e9a50b10159b43b7ebfc21967b0b06fee7ff69c7123e3
      f"
   }
}
```
(base) ekaterinapavlova@MacBook-Air-4 neo-go % curl -X POST  https://rpc
.t5.n3.nspcc.ru:20331 -H 'Content-Type: application/json' -d '{
  "jsonrpc": "2.0",
  "method": "getapplicationlog",
  "params":
  ["61681ce24dffea5481e9a50b10159b43b7ebfc21967b0b06fee7ff69c7123e3f"],
  "id": 1
}' | json_pp
  % Total    % Received % Xferd  Average Speed   Time    Time     Time
  Current
                                 Dload  Upload   Total   Spent    Left
                                 Speed
100   583  100   423  100   160   1424    538 --:--:-- --:--:-- --:--:--
  1969
{
   "id" : 1,
   "jsonrpc" : "2.0",
   "result" : {
      "executions" : [
         {
            "exception" : "at instruction 120 (SYSCALL): can't calculate
             bonus of height unequal (BlockHeight + 1)",
            "gasconsumed" : "198754",
            "notifications" : [],
            "stack" : [
               {
                  "type" : "Integer",
                  "value" : "4704605"
               },
               {
                  "type" : "ByteString",
                  "value" : "KfYYlDe/fxqqqm1yr7o5XLnQ7uk="
               }
            ],
            "trigger" : "Application",
            "vmstate" : "FAULT"
         }
      ],
      "txid" :
      "0x61681ce24dffea5481e9a50b10159b43b7ebfc21967b0b06fee7ff69c7123e3
      f"
   }
}

```
```
(base) ekaterinapavlova@MacBook-Air-4 neo-go % ./bin/neo-go contract
invokefunction -r https://rpc.t5.n3.nspcc.ru:20331 -w ./testnet_wallet
.json 1e6f88377a6c6bc4f683a5fc61eed5645ec5f123 unclaimedGas
e9eed0b95c39baaf726daaaa1a7fbf379418f629 4704605
Enter account NWtk9HYWsf1njtSzA3XNgwZXRtriACcJ9G password >
Warning: FAULT VM state returned from the RPC node: at instruction 120
(SYSCALL): can't calculate bonus of height unequal (BlockHeight + 1).
Use --force flag to send the transaction anyway.
```

Close #3589

Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>

pkg/core/native/native_neo.go

@@ -750,18 +750,21 @@ func makeVoterKey(pub []byte, prealloc ...[]byte) []byte {
 // CalculateBonus calculates amount of gas generated for holding value NEO from start to end block
 // and having voted for active committee member.
 func (n *NEO) CalculateBonus(ic *interop.Context, acc util.Uint160, end uint32) (*big.Int, error) {
-	if ic.Block == nil || end != ic.Block.Index {
-		return nil, errors.New("can't calculate bonus of height unequal (BlockHeight + 1)")
-	}
 	key := makeAccountKey(acc)
 	si := ic.DAO.GetStorageItem(n.ID, key)
 	if si == nil {
-		return nil, storage.ErrKeyNotFound
+		return big.NewInt(0), nil
 	}
 	st, err := state.NEOBalanceFromBytes(si)
 	if err != nil {
 		return nil, err
 	}
+	if st.Balance.Sign() == 0 {
+		return big.NewInt(0), nil
+	}
+	if ic.Block == nil || end != ic.Block.Index {
+		return nil, errors.New("can't calculate bonus of height unequal (BlockHeight + 1)")
+	}
 	return n.calculateBonus(ic.DAO, st, end)
 }
 

pkg/core/native/native_test/neo_test.go

@@ -771,6 +771,43 @@ func TestNEO_CalculateBonus(t *testing.T) {
 	})
 }
 
+func TestNEO_UnclaimedGas(t *testing.T) {
+	neoValidatorsInvoker := newNeoValidatorsClient(t)
+	e := neoValidatorsInvoker.Executor
+
+	acc := neoValidatorsInvoker.WithSigners(e.NewAccount(t))
+	accH := acc.Signers[0].ScriptHash()
+
+	t.Run("non-existing account", func(t *testing.T) {
+		// non-existing account, should return zero unclaimed GAS.
+		acc.Invoke(t, 0, "unclaimedGas", util.Uint160{}, 1)
+	})
+
+	t.Run("zero balance", func(t *testing.T) {
+		initialGASBalance := e.Chain.GetUtilityTokenBalance(accH)
+		h := acc.Invoke(t, true, "transfer", accH, accH, 0, nil)
+		claimTx, _ := e.GetTransaction(t, h)
+		e.CheckGASBalance(t, accH, big.NewInt(initialGASBalance.Int64()-claimTx.SystemFee-claimTx.NetworkFee))
+		acc.Invoke(t, 0, "unclaimedGas", accH, 1)
+	})
+
+	t.Run("unclaimed GAS", func(t *testing.T) {
+		amount := 100
+		defaultGASParBlock := 5
+		rewardDistance := 10
+		neoValidatorsInvoker.Invoke(t, true, "transfer", e.Validator.ScriptHash(), accH, amount, nil)
+
+		for range rewardDistance - 1 {
+			e.AddNewBlock(t)
+		}
+		expectedGas := int64(amount*rewardDistance*defaultGASParBlock) / int64(rewardDistance)
+		acc.Invoke(t, expectedGas, "unclaimedGas", accH, e.Chain.BlockHeight()+1)
+
+		// Test for failure when height does not match (height + 1)
+		acc.InvokeFail(t, "can't calculate bonus of height unequal (BlockHeight + 1)", "unclaimedGas", accH, e.Chain.BlockHeight())
+	})
+}
+
 func TestNEO_GetCandidates(t *testing.T) {
 	neoCommitteeInvoker := newNeoCommitteeClient(t, 100_0000_0000)
 	neoValidatorsInvoker := neoCommitteeInvoker.WithSigners(neoCommitteeInvoker.Validator)