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
```

vRecv >> resp;
        CBlock block;
        bool fBlockRead = false;
        {
            LOCK(cs_main);
            map<uint256, pair<NodeId, list<QueuedBlock>::iterator> >::iterator it = mapBlocksInFlight.find(resp.blockhash);
            if (it == mapBlocksInFlight.end() || !it->second.second->partialBlock ||
                    it->second.first != pfrom->GetId()) {
                LogPrint("net", "Peer %d sent us block transactions for block we weren't expecting\n", pfrom->id);
                return true;
            }
            PartiallyDownloadedBlock& partialBlock = *it->second.second->partialBlock;
            ReadStatus status = partialBlock.FillBlock(block, resp.txn);
            if (status == READ_STATUS_INVALID) {
                MarkBlockAsReceived(resp.blockhash); 
                Misbehaving(pfrom->GetId(), 100);
                LogPrintf("Peer %d sent us invalid compact block/non-matching block transactions\n", pfrom->id);
                return true;
            } else if (status == READ_STATUS_FAILED) {
                
                std::vector<CInv> invs;
                invs.push_back(CInv(MSG_BLOCK | GetFetchFlags(pfrom, chainActive.Tip(), chainparams.GetConsensus()), resp.blockhash));
                pfrom->PushMessage(NetMsgType::GETDATA, invs);
            } else
                fBlockRead = true;
        } 
        if (fBlockRead) {
            CValidationState state;
            ProcessNewBlock(state, chainparams, pfrom, &block, false, NULL);
            int nDoS;