Blender Git Loki

Fix crash in asset browser when switching from file browser

When the file browser is in asset browser mode, it sets the callback
`filelist->prepare_filter_fn` to an asset browser specific function. This
function will segfault if there is no current asset library. Switching back
from asset browser to file browser would not reset that callback to
`NULL`, causing it to be called and crash Blender. This is now fixed.