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
```

pindexNewTip = chainActive.Tip();
            pindexFork = chainActive.FindFork(pindexOldTip);
            fInitialDownload = IsInitialBlockDownload();
        }
        
        
        
        
        for(std::shared_ptr<const CTransaction> tx : txConflicted)
        {
            GetMainSignals().SyncTransaction(*tx, pindexNewTip, CMainSignals::SYNC_TRANSACTION_NOT_IN_BLOCK);
        }
        
        for(unsigned int i = 0; i < txChanged.size(); i++)
            GetMainSignals().SyncTransaction(std::get<0>(txChanged[i]), std::get<1>(txChanged[i]), std::get<2>(txChanged[i]));
        
        GetMainSignals().UpdatedBlockTip(pindexNewTip, pindexFork, fInitialDownload);
        
        if (pindexFork != pindexNewTip) {
            uiInterface.NotifyBlockTip(fInitialDownload, pindexNewTip);
        }
    } while (pindexNewTip != pindexMostWork);
    CheckBlockIndex(chainparams.GetConsensus());
    
    if (!FlushStateToDisk(state, FLUSH_STATE_PERIODIC)) {
        return false;
    }
    return true;
}
bool PreciousBlock(CValidationState& state, const CChainParams& params, CBlockIndex *pindex)