krbd: don't segfault if images are unmapped concurrently

krbd: don't segfault if images are unmapped concurrently

"rbd map c" can die from a NULL dereference on any of this_pool,
this_image or this_snap in wait_for_udev_add():

    <image a is mapped>
    rbd map c
                                    rbd map b
                                    rbd unmap a
                                    rbd unmap b

However unlikely, this segfault is triggered by the rbd/concurrent.sh
workunit on a regular basis.

Similarly, "rbd showmapped" can die if an image to be listed is
unmapped.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>