Fix cluster-fuzz regression with Workers when serializing empty string

BUG=chromium:503991
R=jochen@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1210623002

Cr-Commit-Position: refs/heads/master@{#29263}

src/d8.cc

@@ -1538,8 +1538,10 @@ void SerializationData::WriteTag(SerializationTag tag) { data.Add(tag); }
 
 
 void SerializationData::WriteMemory(const void* p, int length) {
-  i::Vector<uint8_t> block = data.AddBlock(0, length);
-  memcpy(&block[0], p, length);
+  if (length > 0) {
+    i::Vector<uint8_t> block = data.AddBlock(0, length);
+    memcpy(&block[0], p, length);
+  }
 }
 
 

test/mjsunit/regress/regress-crbug-503991.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+if (this.Worker) {
+  __v_3 = "";
+  function __f_1() {}
+  var __v_6 = new Worker(__f_1);
+  __v_6.postMessage(__v_3);
+}