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
```

{
    string strStatusBar;
    string strRPC;
    string strGUI;
    const string uiAlertSeperator = "<hr />";
    if (!CLIENT_VERSION_IS_RELEASE) {
        strStatusBar = "This is a pre-release test build - use at your own risk - do not use for mining or merchant applications";
        strGUI = _("This is a pre-release test build - use at your own risk - do not use for mining or merchant applications");
    }
    if (GetBoolArg("-testsafemode", DEFAULT_TESTSAFEMODE))
        strStatusBar = strRPC = strGUI = "testsafemode enabled";
    
    if (strMiscWarning != "")
    {
        strStatusBar = strMiscWarning;
        strGUI += (strGUI.empty() ? "" : uiAlertSeperator) + strMiscWarning;
    }
    if (fLargeWorkForkFound)
    {
        strStatusBar = strRPC = "Warning: The network does not appear to fully agree! Some miners appear to be experiencing issues.";
        strGUI += (strGUI.empty() ? "" : uiAlertSeperator) + _("Warning: The network does not appear to fully agree! Some miners appear to be experiencing issues.");
    }
    else if (fLargeWorkInvalidChainFound)
    {
        strStatusBar = strRPC = "Warning: We do not appear to fully agree with our peers! You may need to upgrade, or other nodes may need to upgrade.";
        strGUI += (strGUI.empty() ? "" : uiAlertSeperator) + _("Warning: We do not appear to fully agree with our peers! You may need to upgrade, or other nodes may need to upgrade.");