Crash would happen when a linked ID would become missing, that was
'pre-declared' and used only once as a 'weak link' in another library
stored before the one it came from.
In that case, the place-holder generated in read code would be freed in
`read_library_clear_weak_links`, when handling its 'owner' library, but
since all previous libraries in the list had already been 'lib_linked'
and their filedata (and related libmap) freed, the update of the libmaps
in `read_library_clear_weak_links` would not apply to data from those
previous libraries, leading to ID pointers there pointing to freed
memory.
This fix should also be backported to 2.93.
Bastien Montagne
f3be8e66d7