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
```

}
                PartiallyDownloadedBlock& partialBlock = *(*queuedBlockIt)->partialBlock;
                ReadStatus status = partialBlock.InitData(cmpctblock);
                if (status == READ_STATUS_INVALID) {
                    MarkBlockAsReceived(pindex->GetBlockHash()); 
                    Misbehaving(pfrom->GetId(), 100);
                    LogPrintf("Peer %d sent us invalid compact block\n", pfrom->id);
                    return true;
                } else if (status == READ_STATUS_FAILED) {
                    
                    std::vector<CInv> vInv(1);
                    vInv[0] = CInv(MSG_BLOCK | GetFetchFlags(pfrom, pindex->pprev, chainparams.GetConsensus()), cmpctblock.header.GetHash());
                    pfrom->PushMessage(NetMsgType::GETDATA, vInv);
                    return true;
                }
                if (!fAlreadyInFlight && mapBlocksInFlight.size() == 1 && pindex->pprev->IsValid(BLOCK_VALID_CHAIN)) {
                    
                    
                    MaybeSetPeerAsAnnouncingHeaderAndIDs(nodestate, pfrom, connman);
                }
                BlockTransactionsRequest req;
                for (size_t i = 0; i < cmpctblock.BlockTxCount(); i++) {
                    if (!partialBlock.IsTxAvailable(i))
                        req.indexes.push_back(i);
                }
                if (req.indexes.empty()) {
                    
                    BlockTransactions txn;
                    txn.blockhash = cmpctblock.header.GetHash();
                    CDataStream blockTxnMsg(SER_NETWORK, PROTOCOL_VERSION);