sheep/ops.c: fix problem caused by uninitialized backend storage in gateway

when a new gateway(never joined a cluster before) joined a existing
clsuter. `dog vdi object location` causes the new gateway exist
because its backend storage is not initialized.
this patch solves this problems by checking whether a node is gateway
in `local_oid_exist` fucntion. simple return `SD_RES_NO_OBJ` to avoid
using uninitialized pointer.

Signed-off-by: Jinzhi Chen <nxtjinzhi@gmail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>