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
```

const uint256 &hashToAnnounce = pto->vBlockHashesToAnnounce.back();
                    BlockMap::iterator mi = mapBlockIndex.find(hashToAnnounce);
                    assert(mi != mapBlockIndex.end());
                    CBlockIndex *pindex = mi->second;
                    
                    
                    
                    if (chainActive[pindex->nHeight] != pindex) {
                        LogPrint("net", "Announcing block %s not on main chain (tip=%s)\n",
                            hashToAnnounce.ToString(), chainActive.Tip()->GetBlockHash().ToString());
                    }
                    
                    if (!PeerHasHeader(&state, pindex)) {
                        pto->PushInventory(CInv(MSG_BLOCK, hashToAnnounce));
                        LogPrint("net", "%s: sending inv peer=%d hash=%s\n", __func__,
                            pto->id, hashToAnnounce.ToString());
                    }
                }
            }
            pto->vBlockHashesToAnnounce.clear();
        }
        
        
        
        vector<CInv> vInv;
        {
            LOCK(pto->cs_inventory);
            vInv.reserve(std::max<size_t>(pto->vInventoryBlockToSend.size(), INVENTORY_BROADCAST_MAX));
            
            BOOST_FOREACH(const uint256& hash, pto->vInventoryBlockToSend) {