stream: Fix early end in Writables on zero-length writes

Doing this causes problems:

    z.write(Buffer(0));
    z.end();

Fix by not ending Writable streams while they're still in the process of
writing something.

lib/_stream_writable.js

@@ -327,7 +327,10 @@ Writable.prototype.end = function(chunk, encoding, cb) {
 };
 
 function finishMaybe(stream, state) {
-  if (state.ending && state.length === 0 && !state.finished) {
+  if (state.ending &&
+      state.length === 0 &&
+      !state.finished &&
+      !state.writing) {
     state.finished = true;
     stream.emit('finish');
   }

test/simple/test-stream2-writable.js

@@ -326,3 +326,23 @@ test('end(chunk) two times is an error', function(t) {
     t.end();
   });
 });
+
+test('dont end while writing', function(t) {
+  var w = new W();
+  var wrote = false;
+  w._write = function(chunk, e, cb) {
+    assert(!this.writing);
+    wrote = true;
+    this.writing = true;
+    setTimeout(function() {
+      this.writing = false;
+      cb();
+    });
+  };
+  w.on('finish', function() {
+    assert(wrote);
+    t.end();
+  });
+  w.write(Buffer(0));
+  w.end();
+});

test/simple/test-zlib-zero-byte.js

@@ -0,0 +1,48 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+
+var zlib = require('zlib');
+var gz = zlib.Gzip()
+var emptyBuffer = new Buffer(0);
+var received = 0;
+gz.on('data', function(c) {
+  received += c.length;
+});
+var ended = false;
+gz.on('end', function() {
+  ended = true;
+});
+var finished = false;
+gz.on('finish', function() {
+  finished = true;
+});
+gz.write(emptyBuffer);
+gz.end();
+
+process.on('exit', function() {
+  assert.equal(received, 20);
+  assert(ended);
+  assert(finished);
+  console.log('ok');
+});