In commit c091a0bc we removed
the use of the HASH_ macros for inserting into
buf_pool.page_hash, or accessing buf_page_t::hash.

However, the binary buddy allocator for block->page.zip.data would
still use the HASH_ macros. HASH_INSERT and not HASH_DELETE would reset
the next-block pointer to the null pointer. Our replacement of
HASH_DELETE() will reset the next-block pointer, and the replacement of
HASH_INSERT() assumes that the pointer is the null pointer.

buf_LRU_block_free_non_file_page(): Assert that the next-block pointer
is the null pointer.

buf_buddy_block_free(): Reset the pointer before invoking
buf_LRU_block_free_non_file_page(). Without this, the added
assertion would fail in the test encryption.innochecksum.