camera: try to fix double free on descruction

Sometimes after the integration tests ran on destruction I would get
double free of the callbacks. Presumably this was because a callback is
being deleted while being executed.

plugins/camera/camera_impl.cpp

@@ -22,6 +22,26 @@ CameraImpl::CameraImpl(System &system) : PluginImplBase(system)
 CameraImpl::~CameraImpl()
 {
     _parent->unregister_plugin(this);
+
+    {
+        std::lock_guard<std::mutex> lock(_status.mutex);
+        _status.callback = nullptr;
+    }
+
+    {
+        std::lock_guard<std::mutex> lock(_get_mode.mutex);
+        _get_mode.callback = nullptr;
+    }
+
+    {
+        std::lock_guard<std::mutex> lock(_capture_info.mutex);
+        _capture_info.callback = nullptr;
+    }
+
+    {
+        std::lock_guard<std::mutex> lock(_video_stream_info.mutex);
+        _video_stream_info.callback = nullptr;
+    }
 }
 
 void CameraImpl::init()