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
```

LOCK(pto->cs_inventory);
            vector<CBlock> vHeaders;
            bool fRevertToInv = ((!state.fPreferHeaders &&
                                 (!state.fPreferHeaderAndIDs || pto->vBlockHashesToAnnounce.size() > 1)) ||
                                pto->vBlockHashesToAnnounce.size() > MAX_BLOCKS_TO_ANNOUNCE);
            CBlockIndex *pBestIndex = NULL; 
            ProcessBlockAvailability(pto->id); 
            if (!fRevertToInv) {
                bool fFoundStartingHeader = false;
                
                
                
                BOOST_FOREACH(const uint256 &hash, pto->vBlockHashesToAnnounce) {
                    BlockMap::iterator mi = mapBlockIndex.find(hash);
                    assert(mi != mapBlockIndex.end());
                    CBlockIndex *pindex = mi->second;
                    if (chainActive[pindex->nHeight] != pindex) {
                        
                        fRevertToInv = true;
                        break;
                    }
                    if (pBestIndex != NULL && pindex->pprev != pBestIndex) {