drivers/char/tty_io.c: Avoid panic when no console is configured.

When no console is configured tty_open tries to call kref_get on a NULL
pointer, return ENODEV instead.
Signed-off-by: Will Newton <will.newton@gmail.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/tty_io.c

@@ -1795,13 +1795,16 @@ retry_open:
 	}
 #endif
 	if (device == MKDEV(TTYAUX_MAJOR, 1)) {
-		driver = tty_driver_kref_get(console_device(&index));
+		struct tty_driver *console_driver = console_device(&index);
+		if (console_driver) {
+			driver = tty_driver_kref_get(console_driver);
 			if (driver) {
 				/* Don't let /dev/console block */
 				filp->f_flags |= O_NONBLOCK;
 				noctty = 1;
 				goto got_driver;
 			}
+		}
 		mutex_unlock(&tty_mutex);
 		return -ENODEV;
 	}