Follow-up to Bug#24346574 PAGE CLEANER THREAD, ASSERT BLOCK->N_POINTERS == 0

Silence the Valgrind warnings on instrumented builds (-DWITH_VALGRIND).

assert_block_ahi_empty_on_init(): A variant of
assert_block_ahi_empty() that declares n_pointers initialized and then
asserts that n_pointers==0.

In Valgrind-instrumented builds, InnoDB declares allocated memory
uninitialized.

storage/innobase/buf/buf0buf.cc

@@ -1498,7 +1498,7 @@ buf_block_init(
 
 	/* This function should only be executed at database startup or by
 	buf_pool_resize(). Either way, adaptive hash index must not exist. */
-	assert_block_ahi_empty(block);
+	assert_block_ahi_empty_on_init(block);
 
 	block->frame = frame;
 
@@ -2246,7 +2246,7 @@ buf_page_realloc(
 		/* This code should only be executed by buf_pool_resize(),
 		while the adaptive hash index is disabled. */
 		assert_block_ahi_empty(block);
-		assert_block_ahi_empty(new_block);
+		assert_block_ahi_empty_on_init(new_block);
 		ut_ad(!block->index);
 		new_block->index	= NULL;
 		new_block->n_hash_helps	= 0;
@@ -3923,7 +3923,7 @@ buf_block_init_low(
 #ifdef BTR_CUR_HASH_ADAPT
 	/* No adaptive hash index entries may point to a previously
 	unused (and now freshly allocated) block. */
-	assert_block_ahi_empty(block);
+	assert_block_ahi_empty_on_init(block);
 	block->index		= NULL;
 
 	block->n_hash_helps	= 0;

storage/innobase/include/buf0buf.h

@@ -1877,11 +1877,16 @@ struct buf_block_t{
 					or btr_search_own_all(). */
 #  define assert_block_ahi_empty(block)					\
 	ut_a(my_atomic_addlint(&(block)->n_pointers, 0) == 0)
+#  define assert_block_ahi_empty_on_init(block) do {			\
+	UNIV_MEM_VALID(&(block)->n_pointers, sizeof (block)->n_pointers); \
+	assert_block_ahi_empty(block);					\
+} while (0)
 #  define assert_block_ahi_valid(block)					\
 	ut_a((block)->index						\
 	     || my_atomic_addlint(&(block)->n_pointers, 0) == 0)
 # else /* UNIV_AHI_DEBUG || UNIV_DEBUG */
 #  define assert_block_ahi_empty(block) /* nothing */
+#  define assert_block_ahi_empty_on_init(block) /* nothing */
 #  define assert_block_ahi_valid(block) /* nothing */
 # endif /* UNIV_AHI_DEBUG || UNIV_DEBUG */
 	unsigned	curr_n_fields:10;/*!< prefix length for hash indexing:
@@ -1901,6 +1906,7 @@ struct buf_block_t{
 	/* @} */
 #else /* BTR_CUR_HASH_ADAPT */
 # define assert_block_ahi_empty(block) /* nothing */
+# define assert_block_ahi_empty_on_init(block) /* nothing */
 # define assert_block_ahi_valid(block) /* nothing */
 #endif /* BTR_CUR_HASH_ADAPT */
 	bool		skip_flush_check;