Fix T79575: Crash loading nested set-scenes

source/blender/blenloader/intern/readfile.c

@@ -6093,6 +6093,14 @@ static bool scene_validate_setscene__liblink(Scene *sce, const int totscene)
   }
 
   for (a = 0, sce_iter = sce; sce_iter->set; sce_iter = sce_iter->set, a++) {
+    /* This runs per library (before each libraries #Main has been joined),
+     * so we can't step into other libraries since `totscene` is only for this library.
+     *
+     * Also, other libraries may not have been linked yet,
+     * while we could check #LIB_TAG_NEED_LINK the library pointer check is sufficient. */
+    if (sce->id.lib != sce_iter->id.lib) {
+      return true;
+    }
     if (sce_iter->flag & SCE_READFILE_LIBLINK_NEED_SETSCENE_CHECK) {
       return true;
     }