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
```

deque<COutPoint> vWorkQueue;
        vector<uint256> vEraseQueue;
        CTransaction tx;
        vRecv >> tx;
        CInv inv(MSG_TX, tx.GetHash());
        pfrom->AddInventoryKnown(inv);
        LOCK(cs_main);
        bool fMissingInputs = false;
        CValidationState state;
        pfrom->setAskFor.erase(inv.hash);
        mapAlreadyAskedFor.erase(inv.hash);
        if (!AlreadyHave(inv) && AcceptToMemoryPool(mempool, state, tx, true, &fMissingInputs)) {
            mempool.check(pcoinsTip);
            RelayTransaction(tx, connman);
            for (unsigned int i = 0; i < tx.vout.size(); i++) {
                vWorkQueue.emplace_back(inv.hash, i);
            }
            pfrom->nLastTXTime = GetTime();
            LogPrint("mempool", "AcceptToMemoryPool: peer=%d: accepted %s (poolsz %u txn, %u kB)\n",
                pfrom->id,
                tx.GetHash().ToString(),
                mempool.size(), mempool.DynamicMemoryUsage() / 1000);
            
            set<NodeId> setMisbehaving;
            while (!vWorkQueue.empty()) {
                auto itByPrev = mapOrphanTransactionsByPrev.find(vWorkQueue.front());
                vWorkQueue.pop_front();
                if (itByPrev == mapOrphanTransactionsByPrev.end())
                    continue;
                for (auto mi = itByPrev->second.begin();