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 (!fCheckBlockIndex) {
        return;
    }
    LOCK(cs_main);
    
    
    
    if (chainActive.Height() < 0) {
        assert(mapBlockIndex.size() <= 1);
        return;
    }
    
    std::multimap<CBlockIndex*,CBlockIndex*> forward;
    for (BlockMap::iterator it = mapBlockIndex.begin(); it != mapBlockIndex.end(); it++) {
        forward.insert(std::make_pair(it->second->pprev, it->second));
    }
    assert(forward.size() == mapBlockIndex.size());
    std::pair<std::multimap<CBlockIndex*,CBlockIndex*>::iterator,std::multimap<CBlockIndex*,CBlockIndex*>::iterator> rangeGenesis = forward.equal_range(NULL);
    CBlockIndex *pindex = rangeGenesis.first->second;
    rangeGenesis.first++;
    assert(rangeGenesis.first == rangeGenesis.second);