consensus: prevent invalid message panic
After 5c2f69bd2c there's an `invalid
message` panic when trying to run 4-nodes private network:
```
=> Try to restore blocks before running node
2020-06-30T08:59:39.696Z INFO no storage version found! creating genesis block
2020-06-30T08:59:39.696Z INFO service is running {"service": "Prometheus", "endpoint": ":20001"}
2020-06-30T08:59:39.696Z INFO service hasn't started since it's disabled {"service": "Pprof"}
_ ____________ __________
/ | / / ____/ __ \ / ____/ __ \
/ |/ / __/ / / / /_____/ / __/ / / /
/ /| / /___/ /_/ /_____/ /_/ / /_/ /
/_/ |_/_____/\____/ \____/\____/
2020-06-30T08:59:39.696Z INFO starting rpc-server {"endpoint": ":30333"}
2020-06-30T08:59:39.696Z INFO node started {"blockHeight": 0, "headerHeight": 0}
/NEO-GO:/
2020-06-30T08:59:39.697Z INFO new peer connected {"addr": "172.200.0.1:20333", "peerCount": 1}
2020-06-30T08:59:39.697Z INFO new peer connected {"addr": "172.200.0.2:46692", "peerCount": 2}
2020-06-30T08:59:39.697Z INFO new peer connected {"addr": "172.200.0.1:54140", "peerCount": 3}
2020-06-30T08:59:39.697Z INFO new peer connected {"addr": "172.200.0.4:42890", "peerCount": 4}
2020-06-30T08:59:39.697Z INFO new peer connected {"addr": "172.200.0.4:20336", "peerCount": 5}
2020-06-30T08:59:39.697Z INFO new peer connected {"addr": "172.200.0.2:20334", "peerCount": 6}
2020-06-30T08:59:39.697Z INFO started protocol {"addr": "172.200.0.4:42890", "userAgent": "/NEO-GO:/", "startHeight": 0, "id": 3797159912}
2020-06-30T08:59:39.697Z WARN peer disconnected {"addr": "172.200.0.4:20336", "reason": "already connected", "peerCount": 5}
2020-06-30T08:59:39.697Z INFO started protocol {"addr": "172.200.0.2:20334", "userAgent": "/NEO-GO:/", "startHeight": 0, "id": 1116452361}
2020-06-30T08:59:39.697Z WARN peer disconnected {"addr": "172.200.0.2:46692", "reason": "already connected", "peerCount": 4}
2020-06-30T08:59:39.697Z WARN peer disconnected {"addr": "172.200.0.1:54140", "reason": "identical node id", "peerCount": 3}
2020-06-30T08:59:39.697Z WARN peer disconnected {"addr": "172.200.0.1:20333", "reason": "identical node id", "peerCount": 2}
2020-06-30T08:59:40.673Z INFO new peer connected {"addr": "172.200.0.3:44948", "peerCount": 3}
2020-06-30T08:59:40.673Z INFO new peer connected {"addr": "172.200.0.3:20335", "peerCount": 4}
2020-06-30T08:59:40.673Z WARN peer disconnected {"addr": "172.200.0.3:20335", "reason": "already connected", "peerCount": 3}
2020-06-30T08:59:40.673Z WARN peer disconnected {"addr": "172.200.0.3:44948", "reason": "unexpected empty payload", "peerCount": 2}
2020-06-30T08:59:40.673Z INFO new peer connected {"addr": "172.200.0.3:44950", "peerCount": 3}
2020-06-30T08:59:40.673Z INFO new peer connected {"addr": "172.200.0.3:20335", "peerCount": 4}
2020-06-30T08:59:40.677Z INFO node reached synchronized state, starting consensus
2020-06-30T08:59:40.677Z INFO started protocol {"addr": "172.200.0.3:44950", "userAgent": "/NEO-GO:/", "startHeight": 0, "id": 2658713129}
2020-06-30T08:59:40.689Z WARN peer disconnected {"addr": "172.200.0.3:20335", "reason": "already connected", "peerCount": 3}
2020-06-30T08:59:40.697Z INFO blockchain persist completed {"persistedBlocks": 0, "persistedKeys": 23, "headerHeight": 0, "blockHeight": 0, "took": "243.634µs"}
2020-06-30T08:59:41.057Z INFO initializing dbft {"height": 1, "view": 0, "index": 0, "role": "Backup"}
2020-06-30T08:59:41.057Z DPANIC invalid message
2020-06-30T08:59:41.058Z DPANIC invalid message
```
Reason: payload wasn't decoded as payload.message isn't null anymore.
Fixed.
995abcf0c3