Fix cluster-fuzz found regression in d8 Workers

v8::Internal::List will DCHECK when indexing out of the array, even if just to
get the address, and the value is never used. So this construct will fail:

    memcpy(p, &data[0], length);

When data is empty and length is 0.

BUG=chromium:505778
R=mstarzinger@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1216853003

Cr-Commit-Position: refs/heads/master@{#29388}

src/d8.cc

@@ -1578,8 +1578,10 @@ SerializationTag SerializationData::ReadTag(int* offset) const {
 
 
 void SerializationData::ReadMemory(void* p, int length, int* offset) const {
-  memcpy(p, &data[*offset], length);
-  (*offset) += length;
+  if (length > 0) {
+    memcpy(p, &data[*offset], length);
+    (*offset) += length;
+  }
 }
 
 

test/mjsunit/regress/regress-crbug-505778.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) {
+  function __f_3() {
+    onmessage = function() {}
+  }
+  var __v_7 = new Worker(__f_3);
+  __v_7.postMessage("");
+}