staging: lustre: lustre: obdclass: obd_mount.c: Fix NULL dereference

OBD_ALLOC_PTR(uuid) invokes kmalloc, which may return NULL. This fix
adds a check before dereferencing such pointer.

Signed-off-by: Kumar Amit Mehta <gmate.amit@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>