Fix unsafe list element access in vsdclient
Fix unsafe list element access in vsdclient

Method delete_subnet() does access element without check
of return from restproxy.get()

Change-Id: If79d254f8b508d40d9d05e02df4def859ee0e4a2

nuagenetworks/nuage-openstack-neutron,nuagenetworks/nuage-openstack-neutron