MT#61856 control_ng: `json_list_iter()` handle `item_callback`

If any caller passes NULL for item callback and
then receives a JSON with a nested object/list/int, this can crash.

Change-Id: I69fbb9fec76305f6e1ed9ea5b25a50a7ee85c454

daemon/control_ng.c

@@ -365,7 +365,10 @@ static const char *json_list_iter(const ng_parser_t *parser, JsonNode *list,
 				err = str_callback(STR_PTR(s), i, arg);
 		}
 		else
-			err = item_callback(parser, n, arg);
+			if (item_callback)
+				err = item_callback(parser, n, arg);
+			else
+				ilog(LOG_DEBUG, "Ignoring non-string value in list");
 		if (err)
 			return err;
 	}