on B-tree index pages.

page_rec_set_next(): Assert that rec != next.

rec_get_next_ptr(), rec_get_next_offs(): On compact pages, assert that
there are at least REC_N_NEW_EXTRA_BYTES + 1 between records.

page_cur_insert_rec_write_log(): Replace a buf_frame_align() call
with ut_align_offset().

page_cur_insert_rec_low(): Assert that current_rec != insert_rec.