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 (nodestate->nBlocksInFlight >= MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
                        
                        break;
                    }
                    uint32_t nFetchFlags = GetFetchFlags(pfrom, pindex->pprev, chainparams.GetConsensus());
                    vGetData.push_back(CInv(MSG_BLOCK | nFetchFlags, pindex->GetBlockHash()));
                    MarkBlockAsInFlight(pfrom->GetId(), pindex->GetBlockHash(), chainparams.GetConsensus(), pindex);
                    LogPrint("net", "Requesting block %s from  peer=%d\n",
                            pindex->GetBlockHash().ToString(), pfrom->id);
                }
                if (vGetData.size() > 1) {
                    LogPrint("net", "Downloading blocks toward %s (%d) via headers direct fetch\n",
                            pindexLast->GetBlockHash().ToString(), pindexLast->nHeight);
                }
                if (vGetData.size() > 0) {
                    if (nodestate->fSupportsDesiredCmpctVersion && vGetData.size() == 1 && mapBlocksInFlight.size() == 1 && pindexLast->pprev->IsValid(BLOCK_VALID_CHAIN)) {