detect 0 length fs writes with tests

lib/fs.js

@@ -208,6 +208,7 @@ fs.write = function (fd, buffer, offset, length, position, callback) {
     offset = 0;
     length = buffer.length;
   }
+  if(!length) return;
 
   binding.write(fd, buffer, offset, length, position, callback || noop);
 };
@@ -221,6 +222,7 @@ fs.writeSync = function (fd, buffer, offset, length, position) {
     offset = 0;
     length = buffer.length;
   }
+  if(!length) return 0;
 
   return binding.write(fd, buffer, offset, length, position);
 };

test/simple/test-fs-write-sync.js

@@ -8,6 +8,10 @@ fn = path.join(common.fixturesDir, 'write.txt');
 
 foo = 'foo'
 var fd = fs.openSync(fn, 'w');
+
+written = fs.writeSync(fd, '');
+assert.strictEqual(0, written);
+
 fs.writeSync(fd, foo);
 
 bar = 'bár'

test/simple/test-fs-write.js

@@ -10,6 +10,9 @@ var found;
 fs.open(fn, 'w', 0644, function (err, fd) {
   if (err) throw err;
   console.log('open done');
+  fs.write(fd, '', 0, 'utf8', function(err, written) {
+      assert.fail('zero length write should not go through to callback');
+  });
   fs.write(fd, expected, 0, "utf8", function (err, written) {
     console.log('write done');
     if (err) throw err;