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.