qom: allow object_get_canonical_path_component without parent

Just return NULL; any callers that cause a change in behavior
would have caused an assertion failure before, so this is safe.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

include/qom/object.h

@@ -1302,6 +1302,7 @@ Object *object_get_internal_root(void);
  *
  * Returns: The final component in the object's canonical path.  The canonical
  * path is the path within the composition tree starting from the root.
+ * %NULL if the object doesn't have a parent (and thus a canonical path).
  */
 gchar *object_get_canonical_path_component(Object *obj);
 

qom/object.c

@@ -1644,8 +1644,9 @@ gchar *object_get_canonical_path_component(Object *obj)
     ObjectProperty *prop = NULL;
     GHashTableIter iter;
 
-    g_assert(obj);
-    g_assert(obj->parent != NULL);
+    if (obj->parent == NULL) {
+        return NULL;
+    }
 
     g_hash_table_iter_init(&iter, obj->parent->properties);
     while (g_hash_table_iter_next(&iter, NULL, (gpointer *)&prop)) {