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 (!tx.IsCoinBase()) {
        txundo.vprevout.reserve(tx.vin.size());
        BOOST_FOREACH(const CTxIn &txin, tx.vin) {
            CCoinsModifier coins = inputs.ModifyCoins(txin.prevout.hash);
            unsigned nPos = txin.prevout.n;
            if (nPos >= coins->vout.size() || coins->vout[nPos].IsNull())
                assert(false);
            
            txundo.vprevout.push_back(CTxInUndo(coins->vout[nPos]));
            coins->Spend(nPos);
            if (coins->vout.size() == 0) {
                CTxInUndo& undo = txundo.vprevout.back();
                undo.nHeight = coins->nHeight;
                undo.fCoinBase = coins->fCoinBase;
                undo.nVersion = coins->nVersion;
            }
        }
    }
    
    inputs.ModifyNewCoins(tx.GetHash(), tx.IsCoinBase())->FromTx(tx, nHeight);
}
void UpdateCoins(const CTransaction& tx, CCoinsViewCache& inputs, int nHeight)