Close program if `urls-source` is invalid
Close program if `urls-source` is invalid

This code should never be reached (only possible if the if-else list
gets out of sync with `urls-source` values in configcontainer).
Nevertheless, if it was reached it would result in a segmentation fault
(`urlcfg` would be NULL).

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