fix #23 Maat_plugin_exdata的回调使用以'\0'结尾的key。

src/inc_internal/Maat_rule_internal.h

@@ -147,8 +147,8 @@ struct Maat_region_inner
 	int district_id;
 	int table_id;
 	int expr_id_cnt;
-	int expr_id_lb;
-	int expr_id_ub;
+	int expr_id_lb;	//low boundary
+	int expr_id_ub;	//up boundary
 	enum MAAT_TABLE_TYPE table_type;
 };