rbd: always set out parameter in qemu_rbd_snap_list

The caller expects psn_tab to be NULL when there are no snapshots or
an error occurs. This results in calling g_free on an invalid address.

Reported-by: Oliver Francke <Oliver@filoo.de>
Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>