Fix callback access in Pepper's UDP socket resource

ppapi/proxy/udp_socket_resource_base.cc

Index: ppapi/proxy/udp_socket_resource_base.cc
diff --git a/ppapi/proxy/udp_socket_resource_base.cc b/ppapi/proxy/udp_socket_resource_base.cc
index b5c977c3a094a2df67f2cc44892a5a33e10150c2..97864d535b2678ccdb191cca0483b6350b69229f 100644
--- a/ppapi/proxy/udp_socket_resource_base.cc
+++ b/ppapi/proxy/udp_socket_resource_base.cc
@@ -320,6 +320,11 @@ void UDPSocketResourceBase::OnPluginMsgPushRecvResult(
 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))