Enable the PPAPI audio test under NaCl

ppapi/native_client/src/shared/ppapi_proxy/plugin_ppb_audio.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "native_client/src/shared/ppapi_proxy/plugin_ppb_audio.h"
 
 #include <pthread.h>
 #include <stdio.h>
 #include <string.h>
 #include <sys/errno.h>
@@ skipping 156 matching lines @@
 // Start of untrusted PPB_Audio functions
 namespace {
 
 PP_Resource Create(PP_Instance instance,
                    PP_Resource config,
                    PPB_Audio_Callback user_callback,
                    void* user_data) {
   DebugPrintf("PPB_Audio::Create: instance=%"NACL_PRId32" config=%"NACL_PRId32
               " user_callback=%p user_data=%p\n",
               instance, config, user_callback, user_data);
+  if (NULL == user_callback) {
+    return kInvalidResourceId;
+  }
   PP_Resource audio_resource;
   // Proxy to browser Create, get audio PP_Resource
   NaClSrpcError srpc_result = PpbAudioRpcClient::PPB_Audio_Create(
       GetMainSrpcChannel(),
       instance,
       config,
       &audio_resource);
   DebugPrintf("PPB_Audio::Create: %s\n", NaClSrpcErrorString(srpc_result));
   if (NACL_SRPC_RESULT_OK != srpc_result) {
     return kInvalidResourceId;
@@ skipping 139 matching lines @@
       GetAs<ppapi_proxy::PluginAudio>(audio_resource);
   if (NULL == audio.get()) {
     // Ignore if no audio_resource -> audio_instance mapping exists,
     // the app may have shutdown audio before StreamCreated() invoked.
     rpc->result = NACL_SRPC_RESULT_OK;
     return;
   }
   audio->StreamCreated(sync_socket, shm, shm_size);
   rpc->result = NACL_SRPC_RESULT_OK;
 }