lib/core/stream: LineIterator use CachedIterator
lib/core/stream: LineIterator use CachedIterator

The logic behind `is_ok` was broken and caused inconsitent results.

Signed-off-by: Jean Privat <51f8b1fa9b424745378826727452997ee2a7c3d7@pryen.org>

privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit