btr_cur_open_at_index_side() is missing some consistency checks

In MDEV-13542, many sources of crash on a corrupted page were fixed, but something in the function btr_cur_open_at_index_side() was missed.

		if (height == ULINT_UNDEFINED) {

			/* We are in the root node */

			height = btr_page_get_level(page);

			ut_a(height >= level);

This assertion could fail if a nonzero parameter level was supplied to the function. That is only the case when the inline wrapper function btr_pcur_open_at_index_side() is invoked with level>0, which in turn is only possible in the following functions, related to InnoDB persistent statistics:

• dict_stats_analyze_index_level()
• dict_stats_analyze_index_for_n_prefix()

It would seem to make sense to remove the parameter level from the normal functions, and to write a special function that would be used when computing persistent statistics.

Furthermore, btr_cur_open_at_index_side() currently fails to return DB_CORRUPTION on PAGE_LEVEL mismatch or when from_left ? !page_has_prev(page) : !page_has_next(page) does not hold.