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
```

{
    {
        LOCK(cs_main);
        bool fRequested = MarkBlockAsReceived(pblock->GetHash());
        fRequested |= fForceProcessing;
        
        CBlockIndex *pindex = NULL;
        bool fNewBlock = false;
        bool ret = AcceptBlock(*pblock, state, chainparams, &pindex, fRequested, dbp, &fNewBlock);
        if (pindex && pfrom) {
            mapBlockSource[pindex->GetBlockHash()] = pfrom->GetId();
            if (fNewBlock) pfrom->nLastBlockTime = GetTime();
        }
        CheckBlockIndex(chainparams.GetConsensus());
        if (!ret)
            return error("%s: AcceptBlock FAILED", __func__);
    }
    NotifyHeaderTip();
    if (!ActivateBestChain(state, chainparams, pblock))
        return error("%s: ActivateBestChain failed", __func__);
    return true;
}
bool TestBlockValidity(CValidationState& state, const CChainParams& chainparams, const CBlock& block, CBlockIndex* pindexPrev, bool fCheckPOW, bool fCheckMerkleRoot)