dns: pass err to callback instead of throwing

Previously, if invalid input is passed to dns methods like dns.reverse,
an error would be thrown. This change instead passes the error to the
callback.

Fixes: nodejs#3882
Fixes: nodejs#3112

lib/dns.js

@@ -230,7 +230,8 @@ function resolver(bindingName) {
     req.hostname = name;
     req.oncomplete = onresolve;
     var err = binding(req, name);
-    if (err) throw errnoException(err, bindingName);
+    if (err)
+      return callback(errnoException(err, bindingName));
     callback.immediately = true;
     return req;
   };

test/parallel/test-dns.js

@@ -90,6 +90,11 @@ assert.doesNotThrow(function() {
   dns.lookup(NaN, noop);
 });
 
+// Make sure that dns.reverse does not throw if ip is invalid
+assert.doesNotThrow(function() {
+  dns.reverse('foo', noop)
+});
+
 /*
  * Make sure that dns.lookup throws if hints does not represent a valid flag.
  * (dns.V4MAPPED | dns.ADDRCONFIG) + 1 is invalid because: