Fix (studio-reported) crash in some rare cases in blendfile read code.

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.

source/blender/blenloader/intern/readfile.c

@@ -5541,11 +5541,15 @@ static void read_libraries(FileData *basefd, ListBase *mainlist)
     }
   }
 
+  for (Main *mainptr = mainl->next; mainptr; mainptr = mainptr->next) {
+    /* Drop weak links for which no data-block was found.
+     * Since this can remap pointers in `libmap` of all libraries, it needs to be performed in its
+     * own loop, before any call to `lib_link_all` (and the freeing of the libraries' filedata). */
+    read_library_clear_weak_links(basefd, mainlist, mainptr);
+  }
+
   Main *main_newid = BKE_main_new();
   for (Main *mainptr = mainl->next; mainptr; mainptr = mainptr->next) {
-    /* Drop weak links for which no data-block was found. */
-    read_library_clear_weak_links(basefd, mainlist, mainptr);
-
     /* Do versioning for newly added linked data-locks. If no data-locks
      * were read from a library versionfile will still be zero and we can
      * skip it. */