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
```

pnode->PushBlockHash(hash);
                }
            }
        });
    }
}
void PeerLogicValidation::BlockChecked(const CBlock& block, const CValidationState& state) {
    LOCK(cs_main);
    const uint256 hash(block.GetHash());
    std::map<uint256, NodeId>::iterator it = mapBlockSource.find(hash);
    int nDoS = 0;
    if (state.IsInvalid(nDoS)) {
        if (it != mapBlockSource.end() && State(it->second)) {
            assert (state.GetRejectCode() < REJECT_INTERNAL); 
            CBlockReject reject = {(unsigned char)state.GetRejectCode(), state.GetRejectReason().substr(0, MAX_REJECT_MESSAGE_LENGTH), hash};
            State(it->second)->rejects.push_back(reject);
            if (nDoS > 0)
                Misbehaving(it->second, nDoS);
        }
    }
    if (it != mapBlockSource.end())
        mapBlockSource.erase(it);
}
bool static AlreadyHave(const CInv& inv) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
{