[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 <[e-mail]>
Cc: Alan Cox <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/char/tty_ioctl.c

···
 			ld = tty_ldisc_ref(tty);
 			switch (arg) {
 			case TCIFLUSH:
-				if (ld->flush_buffer)
+				if (ld && ld->flush_buffer)
 					ld->flush_buffer(tty);
 				break;
 			case TCIOFLUSH:
-				if (ld->flush_buffer)
+				if (ld && ld->flush_buffer)
 					ld->flush_buffer(tty);
 				/* fall through */
 			case TCOFLUSH: