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
```

chainActive.SetTip(pindexNew);
    
    nTimeBestReceived = GetTime();
    mempool.AddTransactionsUpdated(1);
    cvBlockChange.notify_all();
    static bool fWarned = false;
    std::vector<std::string> warningMessages;
    if (!IsInitialBlockDownload())
    {
        int nUpgraded = 0;
        const CBlockIndex* pindex = chainActive.Tip();
        for (int bit = 0; bit < VERSIONBITS_NUM_BITS; bit++) {
            WarningBitsConditionChecker checker(bit);
            ThresholdState state = checker.GetStateFor(pindex, chainParams.GetConsensus(), warningcache[bit]);
            if (state == THRESHOLD_ACTIVE || state == THRESHOLD_LOCKED_IN) {
                if (state == THRESHOLD_ACTIVE) {
                    strMiscWarning = strprintf(_("Warning: unknown new rules activated (versionbit %i)"), bit);
                    if (!fWarned) {
                        AlertNotify(strMiscWarning);
                        fWarned = true;
                    }
                } else {
                    warningMessages.push_back(strprintf("unknown new rules are about to activate (versionbit %i)", bit));
                }
            }
        }