[PATCH] coverity: tty_ldisc_ref return null check

We add a check of the return value of tty_ldisc_ref(), which
is checked 7 out of 8 times, e.g.:

149  		ld = tty_ldisc_ref(tty);
150  		if (ld != NULL) {
151  			if (ld->set_termios)
152  				(ld->set_termios)(tty, &old_termios);
153  			tty_ldisc_deref(ld);
154  		}

This defect was found automatically by Coverity Prevent, a static analysis
tool.

(akpm: presumably `ld' is never NULL.  Oh well)

Signed-off-by: Zaur Kambarov <zkambarov@coverity.com>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Chris Wright <chrisw@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>