kstat: silence "maybe uninitialized" warnings

Firmly in the "shouldn't happen" camp, but at least GCC 7.4 (Ubuntu
18.04) complained about them, and it's easy to shut up, so do so.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17189