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
```

LogPrint("net", "%s sending header-and-ids %s to peer %d\n", __func__,
                            vHeaders.front().GetHash().ToString(), pto->id);
                    
                    CBlock block;
                    assert(ReadBlockFromDisk(block, pBestIndex, consensusParams));
                    CBlockHeaderAndShortTxIDs cmpctblock(block, state.fWantsCmpctWitness);
                    pto->PushMessageWithFlag(state.fWantsCmpctWitness ? 0 : SERIALIZE_TRANSACTION_NO_WITNESS, NetMsgType::CMPCTBLOCK, cmpctblock);
                    state.pindexBestHeaderSent = pBestIndex;
                } else if (state.fPreferHeaders) {
                    if (vHeaders.size() > 1) {
                        LogPrint("net", "%s: %u headers, range (%s, %s), to peer=%d\n", __func__,
                                vHeaders.size(),
                                vHeaders.front().GetHash().ToString(),
                                vHeaders.back().GetHash().ToString(), pto->id);
                    } else {
                        LogPrint("net", "%s: sending header %s to peer=%d\n", __func__,
                                vHeaders.front().GetHash().ToString(), pto->id);
                    }
                    pto->PushMessage(NetMsgType::HEADERS, vHeaders);
                    state.pindexBestHeaderSent = pBestIndex;
                } else
                    fRevertToInv = true;
            }
            if (fRevertToInv) {
                
                
                
                if (!pto->vBlockHashesToAnnounce.empty()) {