fix: fix var used before declaration in snap_validate_seed

Fix use of variable declared in conditional branch and used in parent
scope in snap_validate_seed. This would affect binary for images without
kernel and using "set -u". (LP: #2037338)

live-build/functions

@@ -767,9 +767,10 @@ snap_preseed() {
 
 snap_validate_seed() {
     local CHROOT_ROOT=$1
+    local kern_major_min=undefined
 
     if [ -e ${CHROOT_ROOT}/boot/vmlinuz ]; then
-        local kern_major_min=$(readlink --canonicalize --no-newline ${CHROOT_ROOT}/boot/vmlinuz | grep  --extended-regexp --only-matching --max-count 1 '[0-9]+\.[0-9]+')
+        kern_major_min=$(readlink --canonicalize --no-newline ${CHROOT_ROOT}/boot/vmlinuz | grep  --extended-regexp --only-matching --max-count 1 '[0-9]+\.[0-9]+')
         if [ -d /usr/share/livecd-rootfs/live-build/apparmor/${kern_major_min} ]; then
             # if an Ubuntu version has different kernel apparmor features between LTS and HWE kernels
             # a snap pre-seeding issue can occur, where the incorrect apparmor features are reported