giochannel: Don't store negative line_term_len in GIOChannel struct

Adding test coverage indicated that this was another bug in 0cc11f74.

Fixes: 0cc11f74 "giochannel: Forbid very long line terminator strings"
Resolves: https://gitlab.gnome.org/GNOME/glib/-/issues/2323


Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 5dc8b001)