bsdinstall: fix prepopulating the ZFS disk menu with ZFSBOOT_DISKS

If the ZFSBOOT_DISKS variable is set to one or more disk names, then
those disks should be preselected in the disk menu. However, the code
wasn't correctly setting the variable, leaving all disks unselected.

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: dteske
Differential Revision: https://reviews.freebsd.org/D35331

(cherry picked from commit caf73e58579343fc1661f43554b30115bb940535)

Fix a memory leak from caf73e58579

Don't shadow an already-local variable with another local declaration.

Reported by: dteske
MFC after: 13 days
MFC with: caf73e58579343fc1661f43554b30115bb940535
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D35331

(cherry picked from commit 77d678b7a4075a056be621b46be50377f08bfc9e)