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
```

if (filterrate && txinfo.feeRate.GetFeePerK() < filterrate) {
                        continue;
                    }
                    if (pto->pfilter && !pto->pfilter->IsRelevantAndUpdate(*txinfo.tx)) continue;
                    
                    vInv.push_back(CInv(MSG_TX, hash));
                    nRelayedTransactions++;
                    {
                        
                        while (!vRelayExpiration.empty() && vRelayExpiration.front().first < nNow)
                        {
                            mapRelay.erase(vRelayExpiration.front().second);
                            vRelayExpiration.pop_front();
                        }
                        auto ret = mapRelay.insert(std::make_pair(hash, std::move(txinfo.tx)));
                        if (ret.second) {
                            vRelayExpiration.push_back(std::make_pair(nNow + 15 * 60 * 1000000, ret.first));
                        }
                    }
                    if (vInv.size() == MAX_INV_SZ) {
                        pto->PushMessage(NetMsgType::INV, vInv);
                        vInv.clear();
                    }
                    pto->filterInventoryKnown.insert(hash);
                }
            }
        }
        if (!vInv.empty())
            pto->PushMessage(NetMsgType::INV, vInv);