librbd: create_snap on a closed image segfaults
I wrote silly code, and in reordering it, managed to attempt rbd_snap_create() on an
image that I had rbd_close()d. This caused quite the odd SEGV in the middle of ldout()
(presumably dereferencing garbage in the CephContext).
It would be nice if methods that use the image could do some simple validation before
getting themselves into trouble.