Merge pull request #3368 from hdcmeta/dx11_cleanup

Set g_vertex_manager to nullptr on DX11 backend shutdown

Source/Core/VideoBackends/D3D/main.cpp

@@ -231,6 +231,8 @@ void VideoBackend::Shutdown()
 		delete g_renderer;
 		g_renderer = nullptr;
 		g_texture_cache = nullptr;
+		g_vertex_manager = nullptr;
+		g_perf_query = nullptr;
 	}
 }