Use ASSERT0P() to check that a pointer is NULL.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kay Pedersen <mail@mkwg.de>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Dag-Erling Smørgrav <des@FreeBSD.org>
Closes #15225