buf_LRU_block_free_non_file_page(): ut_ad(block) before dereferencing block.

buf_LRU_block_remove_hashed_page(): Forbid buf_pool_mutex_exit() while
calling buf_buddy_free().  Callers of buf_LRU_block_remove_hashed_page()
assume that the buffer pool mutex will not be released and reacquired.