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
```

vRecv >> cmpctblock;
        LOCK(cs_main);
        if (mapBlockIndex.find(cmpctblock.header.hashPrevBlock) == mapBlockIndex.end()) {
            
            if (!IsInitialBlockDownload())
                pfrom->PushMessage(NetMsgType::GETHEADERS, chainActive.GetLocator(pindexBestHeader), uint256());
            return true;
        }
        CBlockIndex *pindex = NULL;
        CValidationState state;
        if (!AcceptBlockHeader(cmpctblock.header, state, chainparams, &pindex)) {
            int nDoS;
            if (state.IsInvalid(nDoS)) {
                if (nDoS > 0)
                    Misbehaving(pfrom->GetId(), nDoS);
                LogPrintf("Peer %d sent us invalid header via cmpctblock\n", pfrom->id);
                return true;
            }
        }
        
        assert(pindex);
        UpdateBlockAvailability(pfrom->GetId(), pindex->GetBlockHash());
        std::map<uint256, pair<NodeId, list<QueuedBlock>::iterator> >::iterator blockInFlightIt = mapBlocksInFlight.find(pindex->GetBlockHash());
        bool fAlreadyInFlight = blockInFlightIt != mapBlocksInFlight.end();
        if (pindex->nStatus & BLOCK_HAVE_DATA) 
            return true;
        if (pindex->nChainWork <= chainActive.Tip()->nChainWork || 
                pindex->nTx != 0) { 
            if (fAlreadyInFlight) {