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
```

BOOST_FOREACH(CTxMemPool::txiter ancestorIt, setAncestors)
        {
            const uint256 &hashAncestor = ancestorIt->GetTx().GetHash();
            if (setConflicts.count(hashAncestor))
            {
                return state.DoS(10, false,
                                 REJECT_INVALID, "bad-txns-spends-conflicting-tx", false,
                                 strprintf("%s spends conflicting transaction %s",
                                           hash.ToString(),
                                           hashAncestor.ToString()));
            }
        }
        
        
        CAmount nConflictingFees = 0;
        size_t nConflictingSize = 0;
        uint64_t nConflictingCount = 0;
        CTxMemPool::setEntries allConflicting;
        
        
        
        LOCK(pool.cs);
        if (setConflicts.size())
        {
            CFeeRate newFeeRate(nModifiedFees, nSize);
            set<uint256> setConflictsParents;
            const int maxDescendantsToVisit = 100;
            CTxMemPool::setEntries setIterConflicting;
            BOOST_FOREACH(const uint256 &hashConflicting, setConflicts)