use utis::signed_to_s in ttrss_api.cpp::fetch_feeds_per_category
use utis::signed_to_s in ttrss_api.cpp::fetch_feeds_per_category

ttrss has special categories, which are asigned ids below 0.
If using the unsigned to_s() method these result in queries to
nonexistend categories.

der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,travisred/newsbeuter,x4121/newsbeuter,travisred/newsbeuter,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,travisred/newsbeuter,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,x4121/newsbeuter,newsboat/newsboat,newsboat/newsboat,x4121/newsbeuter,der-lyse/newsboat,x4121/newsbeuter,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,travisred/newsbeuter