Fix sparse warning in tty_io:
drivers/char/tty_io.c:1536:34: warning: Using plain integer as NULL pointer
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
 
        spin_lock_irq(¤t->sighand->siglock);
        tty_pgrp = current->signal->tty_old_pgrp;
-       current->signal->tty_old_pgrp = 0;
+       current->signal->tty_old_pgrp = NULL;
        spin_unlock_irq(¤t->sighand->siglock);
        put_pid(tty_pgrp);