Fix cluster-fuzz found regression in d8 Workers.

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index e165aa47e020e377c713872c552d255ca7acf2f4..3b5accb40f4c0870caf408b124850a2fd5e62d15 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -704,6 +704,10 @@ void Shell::WorkerNew(const v8::FunctionCallbackInfo<v8::Value>& args) {
     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);
   }
 }