mklive.sh: check arch validity after the default is set

It's possible to run mklive.sh without the -a argument in which case,
the architecture is supposed to be set to the current one using
xbps-uhelper. This means we have to check whether the architecture is
valid after this default has been set, otherwise we get an empty string
which is not a valid architecture.

mklive.sh.in

@@ -318,11 +318,6 @@ done
 shift $((OPTIND - 1))
 XBPS_REPOSITORY="$XBPS_REPOSITORY --repository=https://repo-default.voidlinux.org/current --repository=https://repo-default.voidlinux.org/current/musl"
 
-case $BASE_ARCH in
-    x86_64*|i686*) ;;
-    *) >&2 echo architecture $BASE_ARCH not supported by mklive.sh; exit 1;;
-esac
-
 # Configure dracut to use overlayfs for the writable overlay.
 BOOT_CMDLINE="$BOOT_CMDLINE rd.live.overlay.overlayfs=1 "
 
@@ -339,6 +334,11 @@ ARCH=$(xbps-uhelper arch)
 : ${BASE_SYSTEM_PKG:=base-system}
 : ${BOOT_TITLE:="Void Linux"}
 
+case $BASE_ARCH in
+    x86_64*|i686*) ;;
+    *) >&2 echo architecture $BASE_ARCH not supported by mklive.sh; exit 1;;
+esac
+
 # Required packages in the image for a working system.
 PACKAGE_LIST="$BASE_SYSTEM_PKG $PACKAGE_LIST"