Fix zdb "Memory fault" found on FreeBSD ZTS (#16332)

Reason: nvlist_free() tries to free sth. which isn't allocted
Solution: init this variable with NULL

Closes #16311
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>