MDEV-14711 Assertion `mode == 16 || mode == 12 || !fix_block->page.file_page_was_freed' failed in buf_page_get_gen (rollback requesting a freed undo page)

Problem:
=======
In buf_cur_optimistic_latch_leaves(), requesting a left block with BTR_GET
after releasing current block. But there is no guarantee that left block
could be still available.

Fix:
====

(1) In btr_cur_optimistic_latch_leaves(), replace the BUF_GET with
BUF_GET_POSSIBLY_FREED for fetching left block.
(2) Once InnoDB acquires left block, it should check FIL_PAGE_NEXT with
current block page number. If not, release cursor->left_block and return
false.