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
```

CInv _inv(MSG_TX, txin.prevout.hash);
                    pfrom->AddInventoryKnown(_inv);
                    if (!AlreadyHave(_inv)) pfrom->AskFor(_inv);
                }
                AddOrphanTx(tx, pfrom->GetId());
                
                unsigned int nMaxOrphanTx = (unsigned int)std::max((int64_t)0, GetArg("-maxorphantx", DEFAULT_MAX_ORPHAN_TRANSACTIONS));
                unsigned int nEvicted = LimitOrphanTxSize(nMaxOrphanTx);
                if (nEvicted > 0)
                    LogPrint("mempool", "mapOrphan overflow, removed %u tx\n", nEvicted);
            } else {
                LogPrint("mempool", "not keeping orphan with rejected parents %s\n",tx.GetHash().ToString());
            }
        } else {
            if (tx.wit.IsNull() && !state.CorruptionPossible()) {
                
                
                
                assert(recentRejects);
                recentRejects->insert(tx.GetHash());
            }
            if (pfrom->fWhitelisted && GetBoolArg("-whitelistforcerelay", DEFAULT_WHITELISTFORCERELAY)) {