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
```

}
                LOCK(pto->cs_filter);
                for (const auto& txinfo : vtxinfo) {
                    const uint256& hash = txinfo.tx->GetHash();
                    CInv inv(MSG_TX, hash);
                    pto->setInventoryTxToSend.erase(hash);
                    if (filterrate) {
                        if (txinfo.feeRate.GetFeePerK() < filterrate)
                            continue;
                    }
                    if (pto->pfilter) {
                        if (!pto->pfilter->IsRelevantAndUpdate(*txinfo.tx)) continue;
                    }
                    pto->filterInventoryKnown.insert(hash);
                    vInv.push_back(inv);
                    if (vInv.size() == MAX_INV_SZ) {
                        pto->PushMessage(NetMsgType::INV, vInv);
                        vInv.clear();
                    }
                }
                pto->timeLastMempoolReq = GetTime();
            }
            
            if (fSendTrickle) {
                
                vector<std::set<uint256>::iterator> vInvTx;
                vInvTx.reserve(pto->setInventoryTxToSend.size());
                for (std::set<uint256>::iterator it = pto->setInventoryTxToSend.begin(); it != pto->setInventoryTxToSend.end(); it++) {
                    vInvTx.push_back(it);
                }