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 (state.IsInvalid(nDoS)) {
                assert (state.GetRejectCode() < REJECT_INTERNAL); 
                pfrom->PushMessage(NetMsgType::REJECT, strCommand, (unsigned char)state.GetRejectCode(),
                                   state.GetRejectReason().substr(0, MAX_REJECT_MESSAGE_LENGTH), block.GetHash());
                if (nDoS > 0) {
                    LOCK(cs_main);
                    Misbehaving(pfrom->GetId(), nDoS);
                }
            }
        }
    }
    else if (strCommand == NetMsgType::HEADERS && !fImporting && !fReindex) 
    {
        std::vector<CBlockHeader> headers;
        
        unsigned int nCount = ReadCompactSize(vRecv);
        if (nCount > MAX_HEADERS_RESULTS) {
            LOCK(cs_main);
            Misbehaving(pfrom->GetId(), 20);
            return error("headers message size = %u", nCount);
        }
        headers.resize(nCount);
        for (unsigned int n = 0; n < nCount; n++) {
            vRecv >> headers[n];
            ReadCompactSize(vRecv); 
        }
        {
        LOCK(cs_main);
        if (nCount == 0) {