Blender Git Loki

Fix failing assert when loading file with untraceable custom asset library

When loading a file with an asset browser open, and it showed a custom asset
library that can't be found currently (e.g. because the file is from somebody
else), the `BLI_assert(0)` in `rna_FileAssetSelectParams_asset_library_get()`
would fail.

There was code to handle this case already, but unlike I thought it didn't run
right after file read. Now it does.