wrong altogether.  tty_port_tty_get can return NULL and it's not checked
in that manner.

Fix that by checking the tty_port_tty_get retval and put tty kref
properly.

http://stanse.fi.muni.cz/Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Greg KH <greg@kroah.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>