checkboxtree: Don't NULL deref when an invalid array is specified
checkboxtree: Don't NULL deref when an invalid array is specified

Invalid in the sense of asking to append to some row that doesn't exist.

Signed-off-by: Arnaldo Carvalho de Melo <293abb6b76d7791c0732cc517d38c4b5c734b87f@redhat.com>

Distrotech/newt,Distrotech/newt,Distrotech/newt