USB: serial: sierra driver indat_callback fix

A crash has been reported with sierra driver on disconnect with
Ubuntu/Lucid distribution based on kernel-2.6.32.
The cause of the crash was determined as "NULL tty pointer was being
referenced" and the NULL pointer was passed by sierra_indat_callback().

This patch modifies sierra_indat_callback() function to check for NULL
tty structure pointer. This modification prevents a crash from happening
when the device is disconnected.

This patch fixes the bug reported in Launchpad:
  https://bugs.launchpad.net/ubuntu/+source/linux/+bug/511157

Signed-off-by: Elina Pasheva <epasheva@sierrawireless.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/sierra.c

@@ -594,14 +594,17 @@ static void sierra_indat_callback(struct urb *urb)
 	} else {
 		if (urb->actual_length) {
 			tty = tty_port_tty_get(&port->port);
-
-			tty_buffer_request_room(tty, urb->actual_length);
-			tty_insert_flip_string(tty, data, urb->actual_length);
-			tty_flip_buffer_push(tty);
-
-			tty_kref_put(tty);
-			usb_serial_debug_data(debug, &port->dev, __func__,
-				urb->actual_length, data);
+			if (tty) {
+				tty_buffer_request_room(tty,
+					urb->actual_length);
+				tty_insert_flip_string(tty, data,
+					urb->actual_length);
+				tty_flip_buffer_push(tty);
+
+				tty_kref_put(tty);
+				usb_serial_debug_data(debug, &port->dev,
+					__func__, urb->actual_length, data);
+			}
 		} else {
 			dev_dbg(&port->dev, "%s: empty read urb"
 				" received\n", __func__);