NEO: clear LastGasPerVote when voting for NULL, fix neo-project#2894 (neo-project#3173)

This value won't be used in any way, so save some bytes of storage.

Signed-off-by: Roman Khimov <roman@nspcc.ru>
Co-authored-by: Shargon <shargon@gmail.com>
Co-authored-by: Vitor Nazário Coelho <vncoelho@gmail.com>

Author: 3 people

src/Neo/SmartContract/Native/NeoToken.cs

@@ -393,6 +393,10 @@ private async ContractTask<bool> Vote(ApplicationEngine engine, UInt160 account,
             {
                 validator_new.Votes += state_account.Balance;
             }
+            else
+            {
+                state_account.LastGasPerVote = 0;
+            }
             engine.SendNotification(Hash, "Vote",
                 new VM.Types.Array(engine.ReferenceCounter) { account.ToArray(), from?.ToArray() ?? StackItem.Null, voteTo?.ToArray() ?? StackItem.Null, state_account.Balance });
             if (gasDistribution is not null)

tests/Neo.UnitTests/SmartContract/Native/UT_NeoToken.cs

@@ -192,11 +192,13 @@ public void Check_Vote_VoteToNull()
             var accountState = snapshot.TryGet(CreateStorageKey(20, from_Account)).GetInteroperable<NeoAccountState>();
             accountState.Balance = 100;
             snapshot.Add(CreateStorageKey(33, ECCurve.Secp256r1.G.ToArray()), new StorageItem(new CandidateState() { Registered = true }));
+            snapshot.Add(CreateStorageKey(23, ECCurve.Secp256r1.G.ToArray()), new StorageItem(new BigInteger(100500)));
             var ret = Check_Vote(snapshot, from_Account, ECCurve.Secp256r1.G.ToArray(), true, persistingBlock);
             ret.Result.Should().BeTrue();
             ret.State.Should().BeTrue();
             accountState = snapshot.TryGet(CreateStorageKey(20, from_Account)).GetInteroperable<NeoAccountState>();
             accountState.VoteTo.Should().Be(ECCurve.Secp256r1.G);
+            accountState.LastGasPerVote.Should().Be(100500);
 
             //from vote to null account G votes becomes 0
             var G_stateValidator = snapshot.GetAndChange(CreateStorageKey(33, ECCurve.Secp256r1.G.ToArray())).GetInteroperable<CandidateState>();
@@ -211,6 +213,7 @@ public void Check_Vote_VoteToNull()
             G_stateValidator.Votes.Should().Be(0);
             accountState = snapshot.TryGet(CreateStorageKey(20, from_Account)).GetInteroperable<NeoAccountState>();
             accountState.VoteTo.Should().Be(null);
+            accountState.LastGasPerVote.Should().Be(0);
         }
 
         [TestMethod]