window-drag: Check whether window is set when hiding tile preview

In the case where we early-out from meta_window_drag_begin(), the effective_drag_window might not be set yet. In this case, we might finalize the object before effective_drag_window is set, leading to a NULL pointer when accessing window->display in hide_tile_preview().

To avoid that crash, add a check whether the window is set already. If no window is set, we can just skip hiding the preview anyway.