MDEV-25483: Shutdown crash during innodb.innodb_buffer_pool_resize_temporary

buf_resize_callback(): Correct an invalid assertion, and enable
the assertion in debug builds only.

Between buf_resize_start() and buf_resize_shutdown(),
srv_shutdown_state must be less than SRV_SHUTDOWN_CLEANUP.

The incorrect assertion had been introduced in
commit 5e62b6a5 (MDEV-16264).
As a result, the server could crash if shutdown was initiated
concurrently with initiating a change of innodb_buffer_pool_size.

storage/innobase/buf/buf0buf.cc

@@ -2322,7 +2322,7 @@ inline void buf_pool_t::resize()
 static void buf_resize_callback(void *)
 {
   DBUG_ENTER("buf_resize_callback");
-  ut_a(srv_shutdown_state == SRV_SHUTDOWN_NONE);
+  ut_ad(srv_shutdown_state < SRV_SHUTDOWN_CLEANUP);
   mysql_mutex_lock(&buf_pool.mutex);
   const auto size= srv_buf_pool_size;
   const bool work= srv_buf_pool_old_size != size;