Assertion failure in page_rec_get_next_low() during innodb_gis.rtree_compress

A debug assertion that was added in order to catch corruption that was reported in MDEV-17983 turns out to be bogus. In the test innodb_gis.rtree_compress,4k, the function is sometimes being invoked by purge for a spatial index root page that is not a leaf page (PAGE_LEVEL is 1). The assertion should be relaxed as follows:

diff --git a/storage/innobase/include/page0page.ic b/storage/innobase/include/page0page.ic

index 6369932db1b..98b518187b5 100644

--- a/storage/innobase/include/page0page.ic

+++ b/storage/innobase/include/page0page.ic

@@ -660,6 +660,7 @@ page_rec_get_next_low(

 	}

 	ut_ad(page_rec_is_infimum(rec)

+	      || (!page_is_leaf(page) && !page_has_prev(page))

 	      || !(rec_get_info_bits(page + offs, comp)

 		   & REC_INFO_MIN_REC_FLAG));

That is, if the page is a leftmost non-leaf page, it may carry a record where the flag is set.

I observed the failure on 10.5, but the assertion was introduced in 10.2.