native: fix Policy's IsBlocked behaviour

Account is blocked when it's in the Policy's storage, not when it's
missing from the Policy storage. Introduced in
bbbc6805a8.

This bug leads to the fact that during native Neo cache initialization
at the last block in the dBFT epoch, all candidates accounts are
"blocked", and thus, stand-by committee and validators are used in the
subsequent new epoch. Close #3424.

This bug may lead to the consequences described in #3273, but it needs
to be confirmed.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

pkg/core/native/policy.go

@@ -319,7 +319,7 @@ func (p *Policy) IsBlocked(dao *dao.Simple, hash util.Uint160) bool {
 	cache := dao.GetROCache(p.ID)
 	if cache == nil {
 		key := append([]byte{blockedAccountPrefix}, hash.BytesBE()...)
-		return dao.GetStorageItem(p.ID, key) == nil
+		return dao.GetStorageItem(p.ID, key) != nil
 	}
 	_, isBlocked := p.isBlockedInternal(cache.(*PolicyCache), hash)
 	return isBlocked