In commit 138cbec5, we
computed an incorrect parameter to page_dir_split_slot(),
leading us to splitting the wrong directory slot, or
an out-of-bounds access when splitting the supremum slot.
This was once caught in the test innodb_gis.kill_server for
inserting records to a clustered index root page.

page_dir_split_slot(): Take the slot as a pointer, instead of
a numeric index.

page_apply_insert_redundant(), page_apply_insert_dynamic():
Rename slot to last_slot, and make owner_slot a pointer.