librbd: reset snaps in rbd_snap_list()

librbd: reset snaps in rbd_snap_list()

in SnapIterator in rbd.pyx, rbd_snap_list_end() is called by
SnapIterator.__dealloc__(). and rbd_snap_list_end() frees
snaps->name if it is not nullptr.  but there is chance that
snaps->name is never initialized after snaps is allocated by
SnapIterator.__init__, in that case, we will free() a wild pointer.

Fixes: http://tracker.ceph.com/issues/37508
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 5431fba5c5c9122418b6a03df4ed89f86fbb35e3)