Fix (studio-reported) crash in collection management code.

Code checking for potential collection loop dependencies can be called
in cases where we cannot guarantee that there is no NULL pointers, so we
need to check those. Was already done for objects.

source/blender/blenkernel/intern/collection.c

@@ -1423,7 +1423,8 @@ static bool collection_instance_find_recursive(Collection *collection,
   }
 
   LISTBASE_FOREACH (CollectionChild *, collection_child, &collection->children) {
-    if (collection_instance_find_recursive(collection_child->collection, instance_collection)) {
+    if (collection_child->collection != NULL &&
+        collection_instance_find_recursive(collection_child->collection, instance_collection)) {
       return true;
     }
   }