FOR SYSTEM_TIME BETWEEN and FROM .. TO work with negative intervals

Note: I'm not sure it's a bug, and even less sure it's important, it just looks odd and most likely will trigger reports from some meticulous users.

MariaDB [test]> select *, row_start, row_end from t1 for system_time from '2018-04-23 02:00:00' to '2018-04-23 01:35:00';

+------+----------------------------+----------------------------+

| i    | row_start                  | row_end                    |

+------+----------------------------+----------------------------+

|    3 | 2018-04-23 01:30:14.208582 | 2038-01-19 05:14:07.999999 |

+------+----------------------------+----------------------------+

1 row in set (0.00 sec)

Same for BETWEEN.

Current documentation just says

BETWEEN start AND end will show all rows that were visible at any point between two specified points in time. It works inclusively, a row visible exactly at start or exactly at end will be shown too.

and about the same for FROM .. TO.

So, technically it doesn't say that start should be earlier than end, although common sense suggests so, and also it's reasonable to expect semantics close to the old operator BETWEEN, which doesn't return anything for backward intervals.