src: fix intermittent SIGSEGV in resolveTxt

Fixes a SIGSEGV by making sure `txt_chunk` is not empty before setting
it on `txt_records`

Fixes: https://github.com/joyent/node/issues/9285
PR-URL: https://github.com/joyent/node/pull/9300
Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>

src/cares_wrap.cc

@@ -633,7 +633,8 @@ class QueryTxtWrap: public QueryWrap {
       }
       txt_chunk->Set(j++, txt);
     }
-    // Push last chunk
+    // Push last chunk if it isn't empty
+    if (!txt_chunk.IsEmpty())
       txt_records->Set(i, txt_chunk);
 
     ares_free_data(txt_out);

test/internet/test-dns-txt-sigsegv.js

@@ -0,0 +1,8 @@
+var common = require('../common');
+var assert = require('assert');
+var dns = require('dns');
+
+dns.resolveTxt('www.microsoft.com', function(err, records) {
+  assert.equal(err, null);
+  assert.equal(records.length, 0);
+});