Due to the tty ldisc code not stopping write operations against a driver
even after a tty has been closed I added a mechanism to hvc_console in my
previous patch to prevent this by nulling out the tty->driver_data in
hvc_close() but I forgot to check tty->driver_data in hvc_write().  Anton
Blanchard got several oops'es from hvc_write() accessing NULL as if it were
a pointer to an hvc_struct usually stored in tty->driver_data.

So this patch checks tty->driver_data in hvc_write() before it is used. 
Hopefully once Alan Cox's patch is checked in ldisc writes won't continue
to happen after tty closes.

Anton Blanchard has tested this patch and is unable to reproduce the oops
with it applied.

Changelog:

drivers/char/hvc_console.c

- Added comment to hvc_close() to explain the reason for NULLing
  tty->driver_data.

- Added check to hvc_write() to verify that tty->driver_data is valid
  (NOT NULL) which would be the case if the write operation was invoked
  after a tty close was initiated on the tty.
Signed-off-by: Ryan S. Arnold <rsa@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>