MDEV-36863 InnoDB: Failing assertion: !block->n_hash_helps after failing to shrink innodb_buffer_pool_size

buf_pool_t::resize(): After failing to shrink the buffer pool,
clear any adaptive hash index fields for blocks that were restored
from buf_pool.withdrawn to buf_pool.free, so that they will be in
the expected state when the blocks will be allocated later.
This fixes a logic conflict between
commit 4dcb1b5 (MDEV-35049) and
commit b692342 (MDEV-29445).

Author: dr-m

storage/innobase/buf/buf0buf.cc

@@ -2078,6 +2078,19 @@ ATTRIBUTE_COLD void buf_pool_t::resize(size_t size, THD *thd) noexcept
       ut_d(b->in_free_list= true);
       ut_ad(b->state() == buf_page_t::NOT_USED);
       b->lock.init();
+#ifdef BTR_CUR_HASH_ADAPT
+      /* Clear the AHI fields, because buf_block_init_low() expects
+      these to be zeroed. These were not cleared when we relocated
+      the block to withdrawn. Had we successfully shrunk the buffer pool,
+      all this virtual memory would have been zeroed or made unaccessible,
+      and on a subsequent buffer pool extension it would be zero again. */
+      buf_block_t *block= reinterpret_cast<buf_block_t*>(b);
+      block->n_hash_helps= 0;
+# if defined UNIV_AHI_DEBUG || defined UNIV_DEBUG
+      block->n_pointers= 0;
+# endif
+      block->index= nullptr;
+#endif
     }
 
     mysql_mutex_unlock(&mutex);