Liboverride/Outliner: Fix assert in Hierarchy view.

Not sure why/when that was changed, but drawing code should not assume
all items in the hierarchy view tree of the Outliner are liboverrides -
it adds e.g. regular local IDs items for the root item of each library
hierarchy of linked liboverrides.

Reported by @dfelinto & @LukasTonne, thanks.

source/blender/editors/space_outliner/outliner_draw.cc

@@ -1988,11 +1988,14 @@ static void outliner_draw_overrides_restrictbuts(Main *bmain,
     }
 
     ID &id = te_id->get_ID();
-    BLI_assert(ID_IS_OVERRIDE_LIBRARY(&id));
-
     if (ID_IS_LINKED(&id)) {
       continue;
     }
+    if (!ID_IS_OVERRIDE_LIBRARY(&id)) {
+      /* Some items may not be liboverrides, e.g. the root item for all linked libraries (see
+       * #TreeDisplayOverrideLibraryHierarchies::build_tree). */
+      continue;
+    }
 
     const bool is_system_override = BKE_lib_override_library_is_system_defined(bmain, &id);
     const BIFIconID icon = is_system_override ? ICON_LIBRARY_DATA_OVERRIDE_NONEDITABLE :