If the tablespace is dropped or truncated after the
space->is_stopping() check in fil_crypt_get_page_throttle_func(),
we would proceed to request the page, and eventually report a fatal
error.

buf_page_get_gen(): Do not retry reading if mode==BUF_GET_POSSIBLY_FREED.

lock_rec_block_validate(): Be prepared for a NULL return value when
invoking buf_page_get_gen() with mode=BUF_GET_POSSIBLY_FREED.