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
```

} else if (waitingfor == -1) {
                
                waitingfor = mapBlocksInFlight[pindex->GetBlockHash()].first;
            }
        }
    }
}
} 
bool GetNodeStateStats(NodeId nodeid, CNodeStateStats &stats) {
    LOCK(cs_main);
    CNodeState *state = State(nodeid);
    if (state == NULL)
        return false;
    stats.nMisbehavior = state->nMisbehavior;
    stats.nSyncHeight = state->pindexBestKnownBlock ? state->pindexBestKnownBlock->nHeight : -1;
    stats.nCommonHeight = state->pindexLastCommonBlock ? state->pindexLastCommonBlock->nHeight : -1;
    BOOST_FOREACH(const QueuedBlock& queue, state->vBlocksInFlight) {
        if (queue.pindex)
            stats.vHeightInFlight.push_back(queue.pindex->nHeight);
    }
    return true;
}
void RegisterNodeSignals(CNodeSignals& nodeSignals)
{
    nodeSignals.ProcessMessages.connect(&ProcessMessages);
    nodeSignals.SendMessages.connect(&SendMessages);
    nodeSignals.InitializeNode.connect(&InitializeNode);
    nodeSignals.FinalizeNode.connect(&FinalizeNode);
}
void UnregisterNodeSignals(CNodeSignals& nodeSignals)