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
```

for (size_t j = 0; j < tx.vin.size(); j++) {
                prevheights[j] = view.AccessCoins(tx.vin[j].prevout.hash)->nHeight;
            }
            
            for (size_t j = 0; j < tx.vin.size(); j++) {
                auto itByPrev = mapOrphanTransactionsByPrev.find(tx.vin[j].prevout);
                if (itByPrev == mapOrphanTransactionsByPrev.end()) continue;
                for (auto mi = itByPrev->second.begin(); mi != itByPrev->second.end(); ++mi) {
                    const CTransaction& orphanTx = (*mi)->second.tx;
                    const uint256& orphanHash = orphanTx.GetHash();
                    vOrphanErase.push_back(orphanHash);
                }
            }
            if (!SequenceLocks(tx, nLockTimeFlags, &prevheights, *pindex)) {
                return state.DoS(100, error("%s: contains a non-BIP68-final transaction", __func__),
                                 REJECT_INVALID, "bad-txns-nonfinal");
            }
        }
        
        
        
        
        nSigOpsCost += GetTransactionSigOpCost(tx, view, flags);
        if (nSigOpsCost > MAX_BLOCK_SIGOPS_COST)
            return state.DoS(100, error("ConnectBlock(): too many sigops"),
                             REJECT_INVALID, "bad-blk-sigops");
        txdata.emplace_back(tx);
        if (!tx.IsCoinBase())
        {
            nFees += view.GetValueIn(tx)-tx.GetValueOut();