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
```

CBlockIndex *pindexMostWork = NULL;
    CBlockIndex *pindexNewTip = NULL;
    std::vector<std::tuple<CTransaction,CBlockIndex*,int>> txChanged;
    if (pblock)
        txChanged.reserve(pblock->vtx.size());
    do {
        txChanged.clear();
        boost::this_thread::interruption_point();
        if (ShutdownRequested())
            break;
        const CBlockIndex *pindexFork;
        std::vector<std::shared_ptr<const CTransaction>> txConflicted;
        bool fInitialDownload;
        {
            LOCK(cs_main);
            CBlockIndex *pindexOldTip = chainActive.Tip();
            if (pindexMostWork == NULL) {
                pindexMostWork = FindMostWorkChain();
            }
            
            if (pindexMostWork == NULL || pindexMostWork == chainActive.Tip())
                return true;
            bool fInvalidFound = false;
            if (!ActivateBestChainStep(state, chainparams, pindexMostWork, pblock && pblock->GetHash() == pindexMostWork->GetBlockHash() ? pblock : NULL, fInvalidFound, txConflicted, txChanged))
                return false;
            if (fInvalidFound) {
                
                pindexMostWork = NULL;
            }