Make it OOB-safe, warn on truncation, always \0-end, abort on error.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-Id: <20200127092414.169796-5-marcandre.lureau@redhat.com>