tui: fix changing between non-LVM and LVM filesystem in bootdisk chooser

tui: fix changing between non-LVM and LVM filesystem in bootdisk chooser

Happens due to a force-unwrap() under the false assumption that the
disk for LVM configurations always exists when switching to a LVM
filesystem.
This fails spectacularly with a panic when switching from e.g. Btrfs to
ext4 in the filesystem chooser.

Fixes: eda9fa0 ("fix #4856: tui: bootdisk: use correct defaults in advanced dialog")
Reported-by: Christian Ebner <c.ebner@proxmox.com>
Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>