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
```

return state.DoS(100, false, REJECT_INVALID, "coinbase");
    
    
    
    const CChainParams& chainparams = Params();
    if (fRequireStandard && tx.nVersion >= 2 && VersionBitsTipState(chainparams.GetConsensus(), Consensus::DEPLOYMENT_CSV) != THRESHOLD_ACTIVE) {
        return state.DoS(0, false, REJECT_NONSTANDARD, "premature-version2-tx");
    }
    
    bool witnessEnabled = IsWitnessEnabled(chainActive.Tip(), Params().GetConsensus());
    if (!GetBoolArg("-prematurewitness",false) && !tx.wit.IsNull() && !witnessEnabled) {
        return state.DoS(0, false, REJECT_NONSTANDARD, "no-witness-yet", true);
    }
    
    string reason;
    if (fRequireStandard && !IsStandardTx(tx, reason, witnessEnabled))
        return state.DoS(0, false, REJECT_NONSTANDARD, reason);
    
    
    
    if (!CheckFinalTx(tx, STANDARD_LOCKTIME_VERIFY_FLAGS))
        return state.DoS(0, false, REJECT_NONSTANDARD, "non-final");
    
    if (pool.exists(hash))
        return state.Invalid(false, REJECT_ALREADY_KNOWN, "txn-already-in-mempool");
    
    set<uint256> setConflicts;
    {
    LOCK(pool.cs); 
    BOOST_FOREACH(const CTxIn &txin, tx.vin)