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
```

if (--nLimit <= 0)
            {
                
                
                LogPrint("net", "  getblocks stopping at limit %d %s\n", pindex->nHeight, pindex->GetBlockHash().ToString());
                pfrom->hashContinue = pindex->GetBlockHash();
                break;
            }
        }
    }
    else if (strCommand == NetMsgType::GETBLOCKTXN)
    {
        BlockTransactionsRequest req;
        vRecv >> req;
        LOCK(cs_main);
        BlockMap::iterator it = mapBlockIndex.find(req.blockhash);
        if (it == mapBlockIndex.end() || !(it->second->nStatus & BLOCK_HAVE_DATA)) {
            LogPrintf("Peer %d sent us a getblocktxn for a block we don't have", pfrom->id);
            return true;
        }
        if (it->second->nHeight < chainActive.Height() - MAX_BLOCKTXN_DEPTH) {
            LogPrint("net", "Peer %d sent us a getblocktxn for a block > %i deep", pfrom->id, MAX_BLOCKTXN_DEPTH);
            return true;
        }
        CBlock block;
        assert(ReadBlockFromDisk(block, it->second, chainparams.GetConsensus()));
        BlockTransactions resp(req);
        for (size_t i = 0; i < req.indexes.size(); i++) {
            if (req.indexes[i] >= block.vtx.size()) {
                Misbehaving(pfrom->GetId(), 100);