of HASH_DELETE().
HASH_ASSERT_VALID(), HASH_INVALIDATE(): New macros, used in HASH_DELETE()
and HASH_SEARCH().
buf_page_init_low(): Add HASH_INVALIDATE(bpage, hash);
buf_page_init(): Call buf_page_init_low() before HASH_INSERT(), so that
bpage->hash will not be invalidated.
passed as TRUE.
Enclose hash_table_t::adaptive and buf_block_t::n_pointers in
#ifdef UNIV_DEBUG.
btr_search_drop_page_hash_index(): Enclose the corruption check
(which depends on buf_block_t::n_pointers) in #ifdef UNIV_DEBUG.
