[PATCH] tty: use NULL for ptrs

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>