dns: occasionally crashing in resolve4 and setServers

[XadillaX]

• Version: v9.0.0-pre (master)
• Platform: Darwin zanarpro 16.6.0 Darwin Kernel Version 16.6.0: Fri Apr 14 16:21:16 PDT 2017; root:xnu-3789.60.24~6/RELEASE_X86_64 x86_64 i386 MacBookPro11,3 Darwin
• Subsystem: c-ares

Here's the code:

// under test folder
const common = require('../common');
const dns = require('dns');

dns.resolve4('google.com', common.mustCall(function(/* err, nameServers */) {
  dns.setServers([ '8.8.8.8' ]);
}));

dns.resolve4('google.com', common.mustCall(function() {
  // do nothing...
}));

And the Node.js will occasionally crash with:

Assertion failed: (ares__is_list_empty(&server->queries_to_server)), function ares__destroy_servers_state, file ../deps/cares/src/ares_destroy.c, line 102.

[XadillaX]

I'm going to fix this issue. I think I know where the problem is, but I need some time to fix it.