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
```

return true;
        }
        CNodeState *nodestate = State(pfrom->GetId());
        
        
        
        
        
        
        
        
        if (mapBlockIndex.find(headers[0].hashPrevBlock) == mapBlockIndex.end() && nCount < MAX_BLOCKS_TO_ANNOUNCE) {
            nodestate->nUnconnectingHeaders++;
            pfrom->PushMessage(NetMsgType::GETHEADERS, chainActive.GetLocator(pindexBestHeader), uint256());
            LogPrint("net", "received header %s: missing prev block %s, sending getheaders (%d) to end (peer=%d, nUnconnectingHeaders=%d)\n",
                    headers[0].GetHash().ToString(),
                    headers[0].hashPrevBlock.ToString(),
                    pindexBestHeader->nHeight,
                    pfrom->id, nodestate->nUnconnectingHeaders);
            
            
            
            UpdateBlockAvailability(pfrom->GetId(), headers.back().GetHash());
            if (nodestate->nUnconnectingHeaders % MAX_UNCONNECTING_HEADERS == 0) {
                Misbehaving(pfrom->GetId(), 20);
            }
            return true;
        }
        CBlockIndex *pindexLast = NULL;
        BOOST_FOREACH(const CBlockHeader& header, headers) {