Fix loading screen crash caused by clearing with a null depth-stencil buffer.
During managed loading screen initialization, the active depth-stencil buffer is temporarily set to null after the loading render target state is changed.
EndDraw restored the saved depth-stencil buffer only after calling Clear(). This meant that, during scene transitions, Clear() could be called while GraphicsDevice.DepthStencilBuffer was still null, even though ClearOptions.DepthBuffer and ClearOptions.Stencil were requested. As a result, the game could crash when the loading screen finished.
Restore the saved depth-stencil buffer before clearing the device so the depth and stencil clear operations always have a valid buffer.