md: Restore guards in mddestroy()

mddestroy() may be invoked on a partially constructed md device.
Restore the guards that handled this prior to commit e91022168101.

Reported by: syzbot+a0ff73f664de8757cfaa@syzkaller.appspotmail.com
Reported by: syzbot+7b4a4824bf81548283ab@syzkaller.appspotmail.com
Reviewed by: kib
Fixes: e91022168101 ("md(4): move type-specific data under union")
Differential Revision: https://reviews.freebsd.org/D51145