TTY: ntty, add one more sanity check

With the previous patch, we fixed another bug where read_buf was freed
while we still was in n_tty_read. We currently check whether read_buf
is NULL at the start of the function. Add one more check after we wake
up from waiting for input.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>