consensus: fix panic during verifyBlock
Issue: panic during mixed 4-nodes consensus setup:
```
2021-03-18T12:01:50.715Z INFO skip change view {"nc": 0, "nf": 3}
2021-03-18T12:01:52.786Z INFO received ChangeView {"validator": 0, "reason": "Timeout", "new view": 1}
2021-03-18T12:01:53.602Z INFO received ChangeView {"validator": 2, "reason": "Timeout", "new view": 1}
2021-03-18T12:01:56.736Z INFO received ChangeView {"validator": 1, "reason": "Timeout", "new view": 1}
2021-03-18T12:01:56.736Z INFO changing dbft view {"height": 3, "view": 1, "index": 3, "role": "Backup"}
2021-03-18T12:02:01.758Z INFO received PrepareRequest {"validator": 2, "tx": 0}
panic: interface conversion: block.Block is nil, not *consensus.neoBlock
goroutine 315 [running]:
github.com/nspcc-dev/neo-go/pkg/consensus.(*service).verifyBlock(0xc000419540, 0x0, 0x0, 0x4)
github.com/nspcc-dev/neo-go/pkg/consensus/consensus.go:427 +0x1306
github.com/nspcc-dev/dbft.(*DBFT).createAndCheckBlock(0xc0001f8840, 0x13f0002)
github.com/nspcc-dev/dbft@v0.0.0-20210302103605-cc75991b7cfb/dbft.go:373 +0x27e
github.com/nspcc-dev/dbft.(*DBFT).onPrepareRequest(0xc0001f8840, 0x13f4378, 0xc0003b8500)
github.com/nspcc-dev/dbft@v0.0.0-20210302103605-cc75991b7cfb/dbft.go:329 +0xdf1
github.com/nspcc-dev/dbft.(*DBFT).OnReceive(0xc0001f8840, 0x13f4378, 0xc0003b8500)
github.com/nspcc-dev/dbft@v0.0.0-20210302103605-cc75991b7cfb/dbft.go:247 +0xe25
github.com/nspcc-dev/neo-go/pkg/consensus.(*service).eventLoop(0xc000419540)
github.com/nspcc-dev/neo-go/pkg/consensus/consensus.go:297 +0x79d
created by github.com/nspcc-dev/neo-go/pkg/consensus.(*service).Start
github.com/nspcc-dev/neo-go/pkg/consensus/consensus.go:249 +0xa5
```
So (*service).verifyBlock is unable to work with nil block.
4809cdf0b0