fs: fix StatWatcher to handle error codes

Previously, fs.watchFile() would call the callback even if the file does
not exist. This is erroneous and is fixed by correctly handling the
error code provided.

Ref: nodejs/node-v0.x-archive#25469
Fixes: nodejs#1745

lib/fs.js

@@ -1278,7 +1278,7 @@ function StatWatcher() {
 
   this._handle.onchange = function(current, previous, newStatus) {
     if (oldStatus === -1 &&
-        newStatus === -1 &&
+        newStatus < 0 &&
         current.nlink === previous.nlink) return;
 
     oldStatus = newStatus;

test/parallel/test-fs-watch-file-nonexistent.js

@@ -0,0 +1,13 @@
+'use strict';
+
+var fs = require('fs');
+
+var filename = '/path/to/file/that/does/not/exist';
+
+fs.watchFile(filename, function() {
+  throw new Error('callback should not be called for non-existent files');
+});
+
+setTimeout(function() {
+  fs.unwatchFile(filename);
+}, 10);