audio: set push device to null when closing

Fixes crash trying to push samples after parameters change.

app/audio/outputmanager.cpp

@@ -79,6 +79,8 @@ void AudioOutputManager::Close()
   if (output_) {
     output_->stop();
 
+    push_device_ = nullptr;
+
     if (device_proxy_.isOpen()) {
       device_proxy_.close();
     }