n_tty: Fix n_tty_write crash when echoing in raw mode

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=4291086b1f081b869c6d79e5b7441633dc3ace00
http://arstechnica.com/security/2014/05/linux-gets-fix-for-code-execution-flaw-that-went-unpatched-since-2009/