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
```

vInv.push_back(CInv(MSG_BLOCK, hash));
                if (vInv.size() == MAX_INV_SZ) {
                    pto->PushMessage(NetMsgType::INV, vInv);
                    vInv.clear();
                }
            }
            pto->vInventoryBlockToSend.clear();
            
            bool fSendTrickle = pto->fWhitelisted;
            if (pto->nNextInvSend < nNow) {
                fSendTrickle = true;
                
                pto->nNextInvSend = PoissonNextSend(nNow, INVENTORY_BROADCAST_INTERVAL >> !pto->fInbound);
            }
            
            if (fSendTrickle) {
                LOCK(pto->cs_filter);
                if (!pto->fRelayTxes) pto->setInventoryTxToSend.clear();
            }
            
            if (fSendTrickle && pto->fSendMempool) {
                auto vtxinfo = mempool.infoAll();
                pto->fSendMempool = false;
                CAmount filterrate = 0;
                {
                    LOCK(pto->cs_feeFilter);
                    filterrate = pto->minFeeFilter;