md: md_unregister_thread should cope with being passed NULL

md: md_unregister_thread should cope with being passed NULL

Mostly md_unregister_thread is only called when we know that the
thread is NULL, but sometimes we need to check first.  It is safer
to put the check inside md_unregister_thread itself.

Signed-off-by: NeilBrown <neilb@suse.de>