Fix cluster-fuzz found regression in d8 Workers.

src/d8.cc

 // Copyright 2012 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.
 
 
 // Defined when linking against shared lib on Windows.
 #if defined(USING_V8_SHARED) && !defined(V8_SHARED)
 #define V8_SHARED
 #endif
 
@@ skipping 686 matching lines @@
 
   {
     base::LockGuard<base::Mutex> lock_guard(&workers_mutex_);
     if (!allow_new_workers_) return;
 
     Worker* worker = new Worker;
     args.This()->SetInternalField(0, External::New(isolate, worker));
     workers_.Add(worker);
 
     String::Utf8Value function_string(args[0]->ToString());
+    if (!*function_string) {
+      Throw(args.GetIsolate(), "Function.prototype.toString failed");
+      return;
+    }
     worker->StartExecuteInThread(isolate, *function_string);
   }
 }
 
 
 void Shell::WorkerPostMessage(const v8::FunctionCallbackInfo<v8::Value>& args) {
   Isolate* isolate = args.GetIsolate();
   HandleScope handle_scope(isolate);
   Local<Context> context = isolate->GetCurrentContext();
 
@@ skipping 1711 matching lines @@
 }
 
 }  // namespace v8
 
 
 #ifndef GOOGLE3
 int main(int argc, char* argv[]) {
   return v8::Shell::Main(argc, argv);
 }
 #endif