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
```

struct QueuedBlock {
        uint256 hash;
        CBlockIndex* pindex;                                     
        bool fValidatedHeaders;                                  
        std::unique_ptr<PartiallyDownloadedBlock> partialBlock;  
    };
    map<uint256, pair<NodeId, list<QueuedBlock>::iterator> > mapBlocksInFlight;
    
    list<NodeId> lNodesAnnouncingHeaderAndIDs;
    
    int nPreferredDownload = 0;
    
    set<CBlockIndex*> setDirtyBlockIndex;
    
    set<int> setDirtyFileInfo;
    
    int nPeersWithValidatedDownloads = 0;
    
    typedef std::map<uint256, std::shared_ptr<const CTransaction>> MapRelay;
    MapRelay mapRelay;
    
    std::deque<std::pair<int64_t, MapRelay::iterator>> vRelayExpiration;
} 
namespace {
struct CBlockReject {
    unsigned char chRejectCode;
    string strRejectReason;
    uint256 hashBlock;
};