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
```

blockTxnMsg << txn;
                    return ProcessMessage(pfrom, NetMsgType::BLOCKTXN, blockTxnMsg, nTimeReceived, chainparams, connman);
                } else {
                    req.blockhash = pindex->GetBlockHash();
                    pfrom->PushMessage(NetMsgType::GETBLOCKTXN, req);
                }
            }
        } else {
            if (fAlreadyInFlight) {
                
                
                std::vector<CInv> vInv(1);
                vInv[0] = CInv(MSG_BLOCK | GetFetchFlags(pfrom, pindex->pprev, chainparams.GetConsensus()), cmpctblock.header.GetHash());
                pfrom->PushMessage(NetMsgType::GETDATA, vInv);
                return true;
            } else {
                
                
                std::vector<CBlock> headers;
                headers.push_back(cmpctblock.header);
                CDataStream vHeadersMsg(SER_NETWORK, PROTOCOL_VERSION);
                vHeadersMsg << headers;
                return ProcessMessage(pfrom, NetMsgType::HEADERS, vHeadersMsg, nTimeReceived, chainparams, connman);
            }
        }
        CheckBlockIndex(chainparams.GetConsensus());
    }
    else if (strCommand == NetMsgType::BLOCKTXN && !fImporting && !fReindex) 
    {
        BlockTransactions resp;