EVENTHANDLER.9: EVENTHANDLER_REGISTER never fails

Since ecdf4409f910 ("Rework the eventhandler locking [...]"),
EVENTHANDLER_REGISTER() can never return NULL.

Suggested by: olce
Reviewed by: olce, ziaee, zlei
Approved by: olce, zlei
Fixes: ecdf4409f910 ("Rework the eventhandler locking [...]")
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55014

(cherry picked from commit 9d4bad45e233ca7d93666ff1feff6282a88f8a9d)