reap_kill_subtree_once(): handle proctree_lock unlock in reap_kill_proc()

Recorded reaper might loose its reaper status, so we should not assert
it, but check and avoid signalling if this happens.

Reported and tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 week
Differential revision: https://reviews.freebsd.org/D35310