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
```

CAmount nValueIn = 0;
        LockPoints lp;
        {
        LOCK(pool.cs);
        CCoinsViewMemPool viewMemPool(pcoinsTip, pool);
        view.SetBackend(viewMemPool);
        
        bool fHadTxInCache = pcoinsTip->HaveCoinsInCache(hash);
        if (view.HaveCoins(hash)) {
            if (!fHadTxInCache)
                vHashTxnToUncache.push_back(hash);
            return state.Invalid(false, REJECT_ALREADY_KNOWN, "txn-already-known");
        }
        
        
        
        BOOST_FOREACH(const CTxIn txin, tx.vin) {
            if (!pcoinsTip->HaveCoinsInCache(txin.prevout.hash))
                vHashTxnToUncache.push_back(txin.prevout.hash);
            if (!view.HaveCoins(txin.prevout.hash)) {
                if (pfMissingInputs)
                    *pfMissingInputs = true;
                return false; 
            }
        }
        
        if (!view.HaveInputs(tx))
            return state.Invalid(false, REJECT_DUPLICATE, "bad-txns-inputs-spent");
        
        view.GetBestBlock();