There is an apparent problem with page_zip_clear_rec().
In btr_cur_optimistic_update() we do this:

	page_cur_delete_rec(page_cursor, index, offsets, mtr);
...
	rec = btr_cur_insert_if_possible(cursor, new_entry, 0/*n_ext*/, mtr);
	ut_a(rec); /* <- We calculated above the insert would fit */

The problem is that page_cur_delete_rec() could fill the modification
log while doing page_zip_clear_rec(), requiring recompression for the
btr_cur_insert_if_possible(). In a pathological case, the data could
fail to recompress.

page_zip_clear_rec(): Leave the page modification log alone. Only
clear the necessary fields.

rb:673 approved by Jimmy Yang