btr_search_drop_page_hash_when_freed(): Check if buf_page_get_gen()
returns NULL.  The page may have been evicted from the buffer pool
between buf_page_peek_if_search_hashed() and buf_page_get_gen(),
because the buffer pool mutex will be released between these two calls.
(Bug #42279)

rb://82 approved by Heikki Tuuri