tty: Fix possible NULL pointer dereference

During processing of echo characters on terminal device,
n_tty_close can happen in parallel with fault inject
testing method and result in a NULL pointer dereference.

Fix this by validating the tty->disc_data inside
continue_process_echoes() and cancel delayed work before
disc_data become NULL.

Change-Id: I4d24a87f2943653b334251c53cdec1c7252a6099
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>