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
```

(GetBlockProofEquivalentTime(*pindexBestHeader, *mi->second, *pindexBestHeader, consensusParams) < nOneMonth);
                        if (!send) {
                            LogPrintf("%s: ignoring request from peer=%i for old block that isn't in the main chain\n", __func__, pfrom->GetId());
                        }
                    }
                }
                
                
                static const int nOneWeek = 7 * 24 * 60 * 60; 
                if (send && connman.OutboundTargetReached(true) && ( ((pindexBestHeader != NULL) && (pindexBestHeader->GetBlockTime() - mi->second->GetBlockTime() > nOneWeek)) || inv.type == MSG_FILTERED_BLOCK) && !pfrom->fWhitelisted)
                {
                    LogPrint("net", "historical block serving limit reached, disconnect peer=%d\n", pfrom->GetId());
                    
                    pfrom->fDisconnect = true;
                    send = false;
                }
                
                
                if (send && (mi->second->nStatus & BLOCK_HAVE_DATA))
                {
                    
                    CBlock block;
                    if (!ReadBlockFromDisk(block, (*mi).second, consensusParams))
                        assert(!"cannot load block from disk");
                    if (inv.type == MSG_BLOCK)
                        pfrom->PushMessageWithFlag(SERIALIZE_TRANSACTION_NO_WITNESS, NetMsgType::BLOCK, block);
                    else if (inv.type == MSG_WITNESS_BLOCK)
                        pfrom->PushMessage(NetMsgType::BLOCK, block);
                    else if (inv.type == MSG_FILTERED_BLOCK)
                    {