Fix a bogus debug assertion in the fix of Bug #57707.

buf_pool_init(): Replace ut_ad(n_instances < MAX_BUFFER_POOLS)
with ut_ad(n_instances <= MAX_BUFFER_POOLS).
(Spotted by Michael Izioumtchenko.)
Add ut_ad(n_instances > 0).

storage/innobase/buf/buf0buf.c

@@ -1295,7 +1295,8 @@ buf_pool_init(
 	ulint		i;
 	const ulint	size	= total_size / n_instances;
 
-	ut_ad(n_instances < MAX_BUFFER_POOLS);
+	ut_ad(n_instances > 0);
+	ut_ad(n_instances <= MAX_BUFFER_POOLS);
 	ut_ad(n_instances == srv_buf_pool_instances);
 
 	/* We create an extra buffer pool instance, this instance is used