Avoid underflow in dialogs formaction
Avoid underflow in dialogs formaction

If `idx == 0`, `idx - 1` wraps around to a high value because `idx` is
an unsigned integer.

der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat