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 (txinfo.tx && txinfo.nTime <= pfrom->timeLastMempoolReq) {
                        pfrom->PushMessageWithFlag(inv.type == MSG_TX ? SERIALIZE_TRANSACTION_NO_WITNESS : 0, NetMsgType::TX, *txinfo.tx);
                        push = true;
                    }
                }
                if (!push) {
                    vNotFound.push_back(inv);
                }
            }
            
            GetMainSignals().Inventory(inv.hash);
            if (inv.type == MSG_BLOCK || inv.type == MSG_FILTERED_BLOCK || inv.type == MSG_CMPCT_BLOCK || inv.type == MSG_WITNESS_BLOCK)
                break;
        }
    }
    pfrom->vRecvGetData.erase(pfrom->vRecvGetData.begin(), it);
    if (!vNotFound.empty()) {
        
        
        
        
        
        
        
        pfrom->PushMessage(NetMsgType::NOTFOUND, vNotFound);
    }
}
uint32_t GetFetchFlags(CNode* pfrom, CBlockIndex* pprev, const Consensus::Params& chainparams) {