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
```

{
    assert(pindexNew->pprev == chainActive.Tip());
    
    int64_t nTime1 = GetTimeMicros();
    CBlock block;
    if (!pblock) {
        if (!ReadBlockFromDisk(block, pindexNew, chainparams.GetConsensus()))
            return AbortNode(state, "Failed to read block");
        pblock = &block;
    }
    
    int64_t nTime2 = GetTimeMicros(); nTimeReadFromDisk += nTime2 - nTime1;
    int64_t nTime3;
    LogPrint("bench", "  - Load block from disk: %.2fms [%.2fs]\n", (nTime2 - nTime1) * 0.001, nTimeReadFromDisk * 0.000001);
    {
        CCoinsViewCache view(pcoinsTip);
        bool rv = ConnectBlock(*pblock, state, pindexNew, view, chainparams);
        GetMainSignals().BlockChecked(*pblock, state);
        if (!rv) {
            if (state.IsInvalid())