It seems p2p-compactblocks.py fails randomly

From: https://travis-ci.org/bitcoin/bitcoin/jobs/163788152 from #8841
The PR is trivial and totally unrelated to compact block

```
p2p-compactblocks.py:
Initializing test directory /tmp/test3yd_2q3h/2
start_node: bitcoind started, waiting for RPC to come up
start_node: RPC succesfully started
MiniNode: Connecting to Bitcoin Node IP # 127.0.0.1:11016
Testing SENDCMPCT p2p message... 
Testing compactblock headers and shortIDs are correct...
Assertion failed: 
Stopping nodes
Not cleaning up dir /tmp/test3yd_2q3h/2
Failed
stderr:
  File "/home/travis/build/bitcoin/bitcoin/qa/rpc-tests/test_framework/test_framework.py", line 151, in main
    self.run_test()
  File "/home/travis/build/bitcoin/bitcoin/build/../qa/rpc-tests/p2p-compactblocks.py", line 614, in run_test
    self.test_compactblock_construction()
  File "/home/travis/build/bitcoin/bitcoin/build/../qa/rpc-tests/p2p-compactblocks.py", line 264, in test_compactblock_construction
    assert(self.test_node.last_cmpctblock is not None)
Pass: False, Duration: 7 s
```

CNodeState *state = State(nodeid);
    assert(state != NULL);
    if (!state->hashLastUnknownBlock.IsNull()) {
        BlockMap::iterator itOld = mapBlockIndex.find(state->hashLastUnknownBlock);
        if (itOld != mapBlockIndex.end() && itOld->second->nChainWork > 0) {
            if (state->pindexBestKnownBlock == NULL || itOld->second->nChainWork >= state->pindexBestKnownBlock->nChainWork)
                state->pindexBestKnownBlock = itOld->second;
            state->hashLastUnknownBlock.SetNull();
        }
    }
}
void UpdateBlockAvailability(NodeId nodeid, const uint256 &hash) {
    CNodeState *state = State(nodeid);
    assert(state != NULL);
    ProcessBlockAvailability(nodeid);
    BlockMap::iterator it = mapBlockIndex.find(hash);
    if (it != mapBlockIndex.end() && it->second->nChainWork > 0) {
        
        if (state->pindexBestKnownBlock == NULL || it->second->nChainWork >= state->pindexBestKnownBlock->nChainWork)
            state->pindexBestKnownBlock = it->second;
    } else {
        
        state->hashLastUnknownBlock = hash;
    }
}
void MaybeSetPeerAsAnnouncingHeaderAndIDs(const CNodeState* nodestate, CNode* pfrom, CConnman& connman) {