MDEV-21371 Assertion failure in page_rec_get_next_low() during innodb_gis.rtree_compress

A debug assertion that was added in
commit ed0793e0
turns out to be too strict. 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).

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));