LibOverride: Fix resync sometimes deleting root ID of other liboverrides.

While this should not happen in theory, very bad/broken/dirty files can
lead to such situations.

So we need to re-ensure valid root IDs after resync (for now, done after
each 'library indirect level' pass of resync, this may not be 100%
bulletproof though, time will say).

Found while investigating Blender studio issues in Snow parkour short.

source/blender/blenkernel/intern/lib_override.c

@@ -2160,6 +2160,10 @@ static void lib_override_library_main_resync_on_library_indirect_level(
 
   BLI_ghash_free(id_roots, NULL, MEM_freeN);
 
+  /* In some fairly rare (and degenerate) cases, some root ID from other liboverrides may have been
+   * freed, and therefore set to NULL. Attempt to fix this as best as possible. */
+  BKE_lib_override_library_main_hierarchy_root_ensure(bmain);
+
   if (do_reports_recursive_resync_timing) {
     reports->duration.lib_overrides_recursive_resync += PIL_check_seconds_timer() - init_time;
   }