Fix crash when creating a pose asset for which the file list entry in
the asset browser is scrolled off-screen. Because of the
off-screen-ness, it wasn't loaded into memory, which eventually caused
an unexpected NULL pointer.
The solution was to use a different function (`filelist_file_find_id`)
that can reliably find the file list entry, after which the cache entry
can be created.
Reviewed by: Severin
Differential Revision: https://developer.blender.org/D14265
5dca3ee6a2