Fix callback access in Pepper's UDP socket resource

ppapi/proxy/udp_socket_resource_base.cc

 // Copyright 2013 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 "ppapi/proxy/udp_socket_resource_base.h"
 
 #include <algorithm>
 #include <cstring>
 
 #include "base/logging.h"
@@ skipping 302 matching lines @@
   read_buffer_ = NULL;
   bytes_to_read_ = -1;
   recvfrom_addr_resource_ = NULL;
 
   RunCallback(recvfrom_callback_, result);
 }
 
 void UDPSocketResourceBase::OnPluginMsgSendToReply(
     const ResourceMessageReplyParams& params,
     int32_t bytes_written) {
+  // This can be empty if the socket was closed, but there are still tasks
+  // to be posted for this resource.
+  if (sendto_callbacks_.empty())
+    return;
+
   scoped_refptr<TrackedCallback> callback = sendto_callbacks_.front();
   sendto_callbacks_.pop();
   if (!TrackedCallback::IsPending(callback))
     return;
 
   if (params.result() == PP_OK)
     RunCallback(callback, bytes_written);
   else
     RunCallback(callback, params.result());
 }
@@ skipping 27 matching lines @@
   if (result == PP_OK && !data.empty())
     memcpy(output_buffer, data.c_str(), data.size());
 
   recvfrom_addr_ = addr;
 
   return result == PP_OK ? static_cast<int32_t>(data.size()) : result;
 }
 
 }  // namespace proxy
 }  // namespace ppapi