dog: fix segfault bug of getting snapshot for "LOCK_STATE_SHARED" VDI.

After iSCSI initiator logins to the iSCSI target of the VDI,
getting snapshot for that volume causes segault.
This patch fixes false loop.

Signed-off-by: Teruaki Ishizaki <ishizaki.teruaki@lab.ntt.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>