Fix core_snap variable scope issue

The prior change to pre-seed the correct base added a case to handle
installation of the core snap.  When that was added it created a case
where the $core_snap variable would not be defined when we reach the
end of the _snap_preseed function and evaluate if $core_snap is defined
by empty.  Previous if the snap to preseed was core or core18 then
$core_snap would be "" so this patch addresses that by ensuring the
variable exists and is empty by default.

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.525.42) UNRELEASED; urgency=medium
+
+  * Fix core_snap variable scope issue
+
+ -- Robert C Jennings <robert.jennings@canonical.com>  Fri, 28 Feb 2020 08:29:57 -0600
+
 livecd-rootfs (2.525.41) bionic; urgency=medium
 
   * Use snap cli rather than custom snap-tool (LP: #1864252)

live-build/functions

@@ -440,6 +440,7 @@ _snap_preseed() {
     fi
 
     # Pre-seed snap's base
+    local core_snap=""
     case $SNAP_NAME in
         snapd)
             # snapd is self-contained, ignore base
@@ -458,7 +459,7 @@ _snap_preseed() {
                 exit 1
             fi
 
-            local core_snap=$(echo "$snap_info" | grep '^base:' | awk '{print $2}')
+            core_snap=$(echo "$snap_info" | grep '^base:' | awk '{print $2}')
 
             # If snap info does not list a base use 'core'
             core_snap=${core_snap:-core}