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