MDEV-25919 fixup: MSAN and Valgrind errors related to statistics

dict_table_close(): Fix a race condition around dict_stats_deinit().
This was not observed; it should have been caught by an assertion.

dict_stats_deinit(): Slightly simplify the code.

ha_innobase::info_low(): If the table is unreadable,
initialize some dummy statistics.

storage/innobase/dict/dict0dict.cc

@@ -225,6 +225,7 @@ void dict_table_close(dict_table_t *table)
     if (table->release())
     {
       table->stats_mutex_lock();
+      if (table->get_ref_count() == 0)
         dict_stats_deinit(table);
       table->stats_mutex_unlock();
     }
@@ -258,6 +259,7 @@ dict_table_close(
       that FLUSH TABLE can be used to forcibly fetch stats from disk if
       they have been manually modified. */
       table->stats_mutex_lock();
+      if (table->get_ref_count() == 0)
         dict_stats_deinit(table);
       table->stats_mutex_unlock();
     }

storage/innobase/handler/ha_innodb.cc

@@ -14532,7 +14532,12 @@ ha_innobase::info_low(
 	DBUG_ASSERT(ib_table->get_ref_count() > 0);
 
 	if (!ib_table->is_readable()) {
+		ib_table->stats_mutex_lock();
 		ib_table->stat_initialized = true;
+		ib_table->stat_n_rows = 0;
+		ib_table->stat_clustered_index_size = 0;
+		ib_table->stat_sum_of_other_index_sizes = 0;
+		ib_table->stats_mutex_unlock();
 	}
 
 	if (flag & HA_STATUS_TIME) {

storage/innobase/include/dict0stats.ic

@@ -175,16 +175,13 @@ dict_stats_deinit(
 	dict_table_t*	table)	/*!< in/out: table */
 {
 	ut_ad(table->stats_mutex_is_owner());
+	ut_ad(table->get_ref_count() == 0);
 
-	ut_a(table->get_ref_count() == 0);
-
+#ifdef HAVE_valgrind
 	if (!table->stat_initialized) {
 		return;
 	}
 
-	table->stat_initialized = FALSE;
-
-#ifdef HAVE_valgrind
 	MEM_UNDEFINED(&table->stat_n_rows, sizeof table->stat_n_rows);
 	MEM_UNDEFINED(&table->stat_clustered_index_size,
 		      sizeof table->stat_clustered_index_size);
@@ -218,4 +215,5 @@ dict_stats_deinit(
 			sizeof(index->stat_n_leaf_pages));
 	}
 #endif /* HAVE_valgrind */
+	table->stat_initialized = FALSE;
 }