ui-repolist: do not return unsigned (negative) value

The function read_agefile() returns time_t, which is a signed datatime.
We should not return unsigned (negative) value here.

Reported-by: Johannes Stezenbach <js@linuxtv.org>
Signed-off-by: Christian Hesse <mail@eworm.de>

ui-repolist.c

@@ -20,7 +20,7 @@ static time_t read_agefile(const char *path)
 
 	if (readfile(path, &buf, &size)) {
 		free(buf);
-		return -1;
+		return 0;
 	}
 
 	if (parse_date(buf, &date_buf) == 0)