Staging: ozwpan: prevent bogus dereference

app_id comes from the network and can't be trusted.  If it's zero then
it will lead to a kernel crash.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Chris Kelly <ckelly@ozmodevices.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>