fix timer double cancel

Author: Thomas

sdk/src/signaling_device/src/signaling_device_impl.cpp

@@ -185,6 +185,7 @@ void SignalingDeviceImpl::deinit() {
   {
     const std::lock_guard<std::mutex> lock(mutex_);
     timer = timer_;
+    timer_ = nullptr;
   }
   if (timer) {
     timer->cancel();